The effect of using tests in test preparation courses has been subject to debate. While some scholars such as Yang and Badger (2015) believe it is a cause of positive washback effect, others argue that this issue is tentative and context-bound (Green, 2007). Therefore, this study investigated the effect of using Mock tests in International English Language Testing System (IELTS) preparation courses on students' overall IELTS scores. Fifty one IELTS students were selected non-randomly through the quota sampling approach out of 76 students at Mahan Language Institute in Birjand, Iran. These participants were distributed into Group 1 (n = 25) and Group 2 (n = 26). A complete IELTS test was administered to ensure that the Groups were homogeneous and to serve as pretest. After 10 sessions of intervention, a different IELTS test was administered as posttest. The results of between subject analysis through independent samples t-test revealed that using Mock tests in the IELTS preparation courses can positively affect the participants scores on IELTS exam. Pedagogical implications are discussed.