en برنامه General پژوهشي Research Vocabulary and collocation may contribute to effective communication. This study attempted to understand whether vocabulary size or collocation knowledge of upper-intermediate Iranian EFL learners better predicts their writing performance. For this purpose, three different tests including vocabulary size test (Nation and Beglar, 2007) collocation knowledge test (Nizonkiza, 2012), and a writing performance task were administered to sixty students. The results, analyzed through linear regression, showed that while both vocabulary size (β = .521, p= .00) and collocation knowledge (β = .268, p= .03) were important factors in EFL learners' writing performance, vocabulary size was a better predictor of the students’ writing performance. Vocabulary size, collocation knowledge, writing, lexical approach 75 98 http://isoedmag.ir/browse.php?a_code=A-10-2-151&slc_lang=en&sid=1 رضا نجاتی 100319475328460072 100319475328460072 Yes حسن آذین 100319475328460073 100319475328460073 No