The present study primarily aimed at investigating the effect of Task-Based Language Teaching (TBLT) on development of the Iranian EFL learners' ESP Reading Comprehension Skills. Moreover, it was aimed at investigating the probable difference between the TBLT-instructed students of Law and Mechanical Engineering with respect to their ESP reading skills, on the one hand, and the probable difference between TBLT-instructed males and females, on the other. In so doing, four groups of 25 participants (including two experimental groups and two control ones) were selected through cluster random sampling from among ESP students majoring in Law and Mechanical Engineering. After a four-week instruction treatment, the post-test was conducted to the participants. The results of the data analysis revealed that the experimental groups significantly performed better than the control groups in the post-test with respect to their reading comprehension scores. Furthermore, the results of independent samples t-test indicated that TBLT has been more effective on the Mechanical Engineering students than the Law students. Finally, the findings of the study were indicative of the fact that TBLT was more effective on females' reading comprehension rather than on males'. Consequently, it can be concluded that TBLT can have a positive effect on students' ESP reading ability. The findings of this study can be employed in different areas of second/foreign language teaching and learning to facilitate and improve the process of language learning.