This study is an attempt to describe, analyse, compare contrast English and Vietnamese root and epistemic modality as realized by modal verbs from Cognitive perspective, more specifically in terms of force dynamic framework. The study is both descriptive and contrastive in nature. The main aim of the research is to find similarities and differences between English and Vietnamese root senses (including obligation, permission, ability and volition) and epistemic senses (including necessity, probability and possibility) of modal verbs from force dynamics. The main data used in this study are taken from the two corpora: one in English with a total of 500,000 words in 91 social science texts and the other in Vietnamese with 500,000 words in 119 social science texts on the ground that (1) it is a rich resource for the researcher to find examples of root and epistemic meanings of modal verbs in English and Vietnamese to serve the purposes of the study and (2) it is the social science field that the researcher often deals with. The texts are research articles published in English and Vietnamese journals respectively in 7 disciplines: education, psychology, social science, economy, linguistics, culture and law, from 2000 upwards. The data collected are then quantitatively and qualitatively analyzed in order to find similarities and differences between English and Vietnamese root and epistemic senses of modal verbs in terms of force dynamics in case that English is considered as a source language and Vietnamese as a comparative one. With the help of the corpusbased analysis the TexSTAT2 programme, frequency occurrences and KWIC (Key Word In Context) concordance of various modals are discussed in order to show their relative importance in expressing root and epistemic senses in the two languages in question. The findings of the study show that both English and Vietnamese writers conceptualizers use the modality of obligation, permission, ability, volition, necessity, probability, and possibility with different force structures and barriers to express their different opinions or attitudes towards the propositions state of affairs or events. It can be inferred from the results of the study that there exists one common core across English and Vietnamese modal verbs, i.e., the force opposition between the Agonist and the Antagonist. The force can be the one which impinges upon the participant or the state of affair or the event, making the situation necessary (e.g. must in English and phải in Vietnamese). The force may be the one that prevents the participant or the situation from taking place (e.g. can’t in English and không thể in Vietnamese). There may be absence of force, or removal of restraint or no barrier so something is possible (though not necessary) (e.g. can in English and có thể in Vietnamese). However, there are a number of differences between the two languages under study when the conceptualizers writers express their own embodied scientific experiences in communication by using various modals with different levels of strength of cognitive, sociophysical and rational forces. One of the typical differences is that in English, low strength forces of modals such as can, could, may, might, predominate over median (will would should ought) and high strength (must have to need cannot), which indicates a tendency for modality to be used to mitigate than to strengthen assertations in academic writing. In contrast, in Vietnamese, high strength modals (phải, cần, không thể) predominate over median (nên, sẽ, muốn, định, toan) and low (có thể), which can be inferred that the Vietnamese writers conceptualizers when writing their papers in social science journals have a tendency of expressing strong obligation and necessity. The overweight of high dynamic value in Vietnamese may be due to the fact that the three major philosophical traditions: Confucianism, Buddhism and Taoism have exerted their influence on the “subjective culture” of the Vietnamese. (cf. Bochner 1986 Marr 1981, cited in Ellis 1994 T.N.Thêm 1998: 25). It is hoped that the findings from this study will make a contribution to further understanding of root and epistemic modality in English compared to Vietnamese in terms of force dynamics and their equivalence and nonequivalence in the expressions of obligation, permission, ability, volition (in sociophysical interactions and relations), and the modality of necessity, probability and possibility (in reasoning domain). Moreover, the findings of similarities and differences between the two languages will be useful in language teaching and learning and translation from English into Vietnamese andor vice versa.