The present study aimed at investigating the possible impact of Zone of Proximal Development (ZPD) on the learners’ performance. Forty-six Iranian university students participated in this study by taking two types of reading comprehension tests (i.e., the main idea and Wh-question type). The researchers used the Winstep program to analyze the data and estimate the learners’ Rasch measure in pre, post, and mediation tests. The study indicated that the ZPD was the main reason for the observed difference between the learners’ performance on pretest (Actual Level of Development or ALD) and posttest (Future Level of Development or FLD). The study also revealed that the learners with lower Rasch measure showed more sensitivity to mediation.