This study attempted to investigate the effect of teaching creative thinking techniques on the development of lexical complexity in EFL students' essays in both timed and untimed conditions. For this quasi-experimental study, 54 female undergraduates of English literature, assigned to two groups of 27 each, were selected from two intact writing classes. In addition to their regular writing class, both groups were taught for 10 extra sessions through an online tool named Padlet. The experimental group (EG) was taught creative thinking techniques and the comparison group (CG) practiced extra process writing activities. Moreover, TOEFL essay questions were administered as pre and posttests. In both timed and untimed essays, three indices of lexical complexity, namely lexical density, lexical sophistication and lexical variation were measured through an automatic analyzing software. Mixed MANOVA showed that practicing creative thinking techniques had a significant effect on improving lexical complexity of EFL students in both conditions. Thus, material developers and EFL writing teachers could benefit from incorporating activities related to these techniques in EFL writing textbooks and also EFL classes, teaching students to develop lexical complexity in their essays.