Background: In response to globalism, many East-Asian countries now include a Medical English course in their undergraduate medical education syllabus. The purpose of this study was to explore the relationship between the related attribute factors of students' motivation to learn medical English through an online modality. Methods: Of 134 eligible fourth-year medical students who participated in an Online Medical English course at a Japanese medical school, 105 were enrolled in this single cohort study. The participants completed pre- and post-course surveys regarding their motivation during the course, including perceived academic control and task value, and their assignment scores. A structural equation model was used to examine the hypothesized relationship of constructs, based on control-value theory. Results: The model showed a good fit for the data (χ 2[df=7] = 1.821, p=0.969, CFI = 1.000, RMSEA < 0.001, SRMR < 0.05, GFI = 0.993, AGFI = 0.980). The latent variables of the perceived course achievement related to the observed variables of academic control and task value scale scores, and negatively predicted willingness for self-study after course completion. In addition, the preference of English as the course language negatively predicted willingness for self-study of medical English. Conclusion: Choice of English as the language of instruction and perceived high course achievement negatively predicted students' motivation for further English self-study after the class. The importance of incorporating the perspective of lifelong learning into the teaching of medical English was recognized.