The relationship between the two constructs of size of vocabulary (number of known words) and depth of knowledge (how well those words are known) is still unclear. The present study investigated the effect of depth and size of word knowledge instruction on descriptive writing performance, lexical aspect, and lexical cohesion of Iranian EFL learners. For this purpose, 40 BA EFL learners were selected from the University of Maragheh. Two descriptive writing tasks were utilized as the pre- and posttest of the study, VLT to measure vocabulary size, WAT to measure vocabulary depth, writing analytic scale, and Hoey’s lexical cohesion framework (1991). Different descriptive and inferential statistical tests were employed in the process of data analysis including a Pearson’s correlation test, Kolmogorov-Smirnov test of normality, and Levene’s test of homogeneity of variances, and independent sample t-test. Findings of the study indicated that the 1) depth of vocabulary was a significant predictor of the learners’ descriptive writing performance; 2) depth of vocabulary was a significant predicator of the vocabulary, and lexical cohesion of the learners’ descriptive writings; 3) size of vocabulary doesn’t play effective role in improving content, organization, vocabulary, and mechanics of descriptive writing; 4) there is no significant correlation between the learners’ descriptive writing performance and their size of vocabulary; 5) the depth of vocabulary was a significant predictor of the lexical cohesion of the learners’ L2 descriptive essays; 6) the size of vocabulary was not a significant predicator of the lexical cohesion of the learners’ L2 descriptive essays.
