This study examined factors affecting international students' willingness to communicate (WTC) in English as a second language (ESL), focusing on second language (L2) self-confidence, acculturation, and motivational types. L2 self-confidence was hypothesized to predict L2 WTC and to mediate the correlation between acculturation and L2 WTC for instrumental motivation learners, but not for integrative ones. Participants were 88 Japanese students studying at a university in California. The results partially confirmed the hypotheses, indicating that both L2 self-confidence and acculturation strongly correlate with L2 WTC, but the mediation effect of L2 self-confidence on the correlation between acculturation and L2 WTC was opposite from the hypothesis, as the mediation effect was significant for integrative motivation learners but not for instrumental ones.