This study investigated the Relationship between Depth of Vocabulary Knowledge and Semantic Set in Iranian EFL Learners’ Vocabulary Used in Writing. The purpose of this study was to see the role of depth of vocabulary knowledge in making appropriate choices among words in a semantic set in EFL learners’ writing performance. 70 lower-intermediate and upper-intermediate learners of English as a foreign language (EFL) from the language learners of English institute in Bandar Abbas, Iran participated in this study. Instruments for the data collection include Word Association Test (WAT), Vocabulary Knowledge Scale (VKS), and four writing tests. In the first phase, using MANOVA statistical procedure for data analysis, the role of depth of vocabulary knowledge in writing performance and appropriate use of words in a semantic set in overall writing quality was investigated. The second part of the study attempted to explore the role of appropriate use of words in a semantic set in the participants’ overall writing performance using independent sample t-test. Finally, attempts were made to identify the role of depth of vocabulary knowledge and appropriate use of words in a semantic set in the overall use of words in the participants’ writings using a two-way ANOVA as the data analysis technique. The results of the data analysis provided empirical evidence supporting that: a) depth of vocabulary knowledge played a fundamental role in appropriate use of words in a semantic set in lower-intermediate level but not in upper-intermediate level, b) depth of vocabulary knowledge has a significant role in overall writing performance only in upper-intermediate level, c) using words appropriately in a semantic set played a significant role in writing performance of the lower-intermediate participants, while the opposite was revealed for the upper-intermediate group, d) the upper-intermediate learners could benefit from their knowledge of depth of vocabulary in using words, in general, correctly and appropriately. The findings promise some pedagogical implications for vocabulary and writing curriculum development, material development, and course designing.