The present study reported on the directionality of the relationships between reading and writing in English and the effects of background knowledge(i.e., English vs. non-English major group) and English proficiency in moderating these relationships. Results of the present study show that a bi-directional and interactive model of reading-writing relationship fit better the English major group, while a reading-to-writing model was better supported by the data for the non-English major group. With regards to the moderating effects of English proficiency, it was found that the strength of the relationships between reading and writing did not significantly differ according to the level of English proficiency in the English major group. However, in the non-English major group, English proficiency significantly moderated the effects of reading on writing, but it did not significantly moderate the effects of writing on reading. Pedagogical implications are also provided.