This study examines the relative contributions of linguistic and affective variables to writing performance among students learning English as a foreign language (EFL) in South Korea. In total, 270 middle school students completed four language tests assessing their reading comprehension, grammar knowledge, and descriptive and argumentative writing skills, as well as a questionnaire on their previous writing instruction, motivation (intrinsic value, utility value, and cognitive/linguistic value), writing apprehension, and self-efficacy. Confirmatory factor analysis and structural equation modeling were performed to examine the predictive value of each predictor of EFL writing. The results revealed that reading comprehension is the most influential contributor to writing proficiency, followed by grammar knowledge and previous writing instruction. Although cognitive/linguistic value and apprehension were found to be significant predictors, the remaining three affective variables--intrinsic value, utility value, and self-efficacy--were not. The findings suggest that improving language-specific knowledge is an essential prerequisite for developing the EFL writing performance of middle school students having limited linguistic knowledge.