This study sought to investigate factors affecting the English writing performance of 30 Korean ESL students overseas. Also, examined were the relationship between L1 and L2 education levels and L2 writing performance, the relationship between English language proficiency and L2 writing performance and between length of residence and L2 writing performance. The participants were asked to write essays in their L1 and L2 along with a questionnaire. To analyze the data, two-way ANOVA and correlation were performed. The results showed there was no significant relation between L1 educational level and L2 writing performance, but there was a significant relation between L2 educational level and L2 writing performance. Also, English language proficiency and L2 writing performance were highly correlated. The length of residence of the participants in the U.S. and their L2 writing performance showed a weak correlation. Similarly, their L1 writing performance was not related to their L2 writing performance. These imply that students need to have adequate proficiency of English to write in their L2, and they need to receive a considerate number of years of formal education in their L2 for the success of L2 writing.