The study investigated whether Korean EFL students' vocabulary used in reading-based writing differed according to writing topic and their reading and writing proficiency. College students enrolled in writing courses (n=95) were asked to write argumentative essays in response to two readings on judging people by appearance (JPA) and disclosing personal information of serious criminals (DPI). These students were divided into high and low proficiency writer groups and into high and low proficiency reader groups according to their writing and reading scores respectively. The students' vocabulary used in writing was then analyzed by VocabProfile, which provided four lexical frequency lists: the first 1000 frequent words (K1) including function words (FW) and content words (CW), the second 1000 frequent words (K2), academic word list (AWL), and off the list words (OLW). The results indicated that the topic JPA produced a higher proportion of K1 and content words, whereas DPI generated more K2 and off the list words. None of the vocabulary profiles, however, significantly differed according to the students’ reading proficiency. In contrast, proficient writers were found to use significantly more K1 and function words than their counterparts. With the topic effect further considered, for JPA, proficient writers used more K1 words and function words whereas less proficient writers used more K2 and off the list words. With regard to DPl, proficient writers were found to use more function words than low proficient writers. Findings are discussed in more detail, along with implications.