The study was to investigate the effects of L2 learning motivation on English ability of Korean university students. One hundred sixty seven Korean university students studying in a university in Seoul, Korea participated in this survey. The correlation analysis between motivations and English ability showed that attitudes toward learning English was the most significantly correlated with English ability, followed by cultural interests, integrativeness, and interests in foreign languages. The t-test revealed that the advanced levels reported the higher motivations in cultural interests, interests in foreign languages, attitudes to learning English, ideal L2 self, and integrativeness than the low levels. In addition, male students showed a higher mean score in ought-to L2 self, while female students showed significantly higher mean scores in international contact, interest in foreign languages, attitudes to learning English, and ideal L2 self. Finally, the regression analysis showed that the causal effects of two variables, attitudes to learning English and cultural interests, on English ability were statistically significant. In addition, males`` English ability was influenced by attitudes to learning English and cultural interests, while females``, by attitudes to learning English and international contacts. The theoretic and pedagogical discussions will be made at the end.