Developing reading comprehension ability is an important aspect in acquisition of a language. The present study focused on improving reading comprehension ability through Task-based Instruction (TBI). TBI is a methodology that develops from a focus on classroom tasks. The participants of the study were 135 Iranian female students at different levels selected from high schools in Isfahan, Iran, through a quasi-experimental design. The participants were divided into four groups, two control groups (CGs) and two experimental groups (EGs). They received a pre-test, the instruction, and a post-test. The participants in EGs were taught through TBI, whereas CGs were exposed to a traditional method. The comparison between CGs and EGs were made through paired sample t-tests. The results revealed that the students in EGs outperformed CGs. The difference between the two grades was also investigated by independent sample t-test. The results showed that students at first-grade outperformed fourth-graders. The findings suggest that using flexible and interactive tasks in English classes improves reading comprehension ability of Iranian EFL learners. This study may have pedagogical implications for practitioners in the field and for syllabus designers to include appropriate tasks in English textbooks.