This study explored the relationship between Korean-speaking English learners’ L1 writing ability, their general English proficiency, and their L2 writing performance. It further investigated whether the relationship among these variables would remain the same depending on the difficulty of the English writing task. One hundred nine Korean learners of English wrote one argumentative essay in Korean and two argumentative essays in English, one on a more difficult topic and the other on an easier topic. The participants’ general English proficiency was measured by their scores on a standardized test. In the analysis using Structural Equation Model (SEM), the present study showed that in general, Korean students’ English writing performance was significantly influenced by their L1 writing ability and by their general English proficiency. When the models were tested separately by English topic, the influences of general English proficiency were weaker for a more difficult topic than for an easier one, while the effect of L1 writing on L2 writing stayed the same regardless of the topic difficulty.