Epistemic modality is described as the speaker's judgment, assessing the degree of the factual proposition, or the speaker's confidence or non-confidence in the truth value of the proposition. The paper focuses on the expression of “thấy”, “nghĩ” predicates in Vietnamese and compares with mental verbs in English. To achieve the research objectives, the paper uses descriptive, statistical and comparative methods to clarify modality in general and epistemic modality in particular. The paper initially contributes to clarifying the definition, category, and identification of epistemic modality in terms of grammar and vocabulary. Next, the paper points out the similarities and differences in the expression of epistemic modality in English and Vietnamese. The results show that the nature of vocabulary and syntax basically exists value of factuality and non-factuality. The most important feature of epistemic modality is subjectivity and objectivity. The speaker "I/Tôi" clearly expresses or implies commitment for what is said. Understanding and applying modality in general and epistemic modality in particular will help learners acquire language and culture better in the process of communication and translation.