The present study investigated the relationships between acculturation and English learning among Korean students at an American university. A survey was conducted with 115 students whose first language is Korean. Data was collected through an acculturation questionnaire, self perceived Korean and English proficiency questionnaire and background questionnaire. ANOVA, Post Hoc Tests, Group comparison and multiple regression were used to analyze the data. The majority of students in the survey chose bicultural adaptation as their strategy for acculturation and did not lose Korean cultural identity although some of them exhibited linguistic assimilation. In the four group comparisons based on acculturation types, students in a high bicultural tendency group reported the greater level of English proficiency. The results of multiple regression showed that students’ age of arrival in the U.S, their level of American acculturation, and gender (male), were important predictors of their English proficiency. Different from the assimilation theory in second language acquisition both integration into American society and positive acceptance of Korean culture and identity seemed to be important processes for their achievement of successful English learning as well as for their successful acculturation. In conclusion, the pedagogical implication of the finding and future research were suggested.