Nowadays, many second/foreign language (L2) academic writing instruction programs place a high premium on pre-writing strategies. The current study examined the effect of software-supported concept mapping on lexical diversity (LD) of English learners’ argumentative essays within a process writing framework. Additionally, the relationship between the learners’ LD and their overall writing quality was investigated. To this end, 53 university English as a foreign language (EFL) undergraduates were assigned to a computer-aided concept mapping (CACM) and a traditional outlining condition over a span of seven weeks. The CACM group was instructed through the graphic organizer software Inspiration®, whereas the comparison group underwent outlining instruction for planning their writing tasks. Measure of textual lexical diversity (MTLD) was used to assess the so-called D values of the assignments. The results revealed that the CACM group outperformed the outlining group in terms of LD scores. Also, no relationship was found between LD and overall quality of the essays. The findings provide L2 researchers and teachers with insights into understanding the use of CACM strategy in process writing. Moreover, exploiting MTLD afforded our experiment the opportunity to counteract potential pitfalls associated with text size. Further implications for the L2 teacher are also discussed.