Abstract: In order to make a change in the way students are prepared to meet the demands of the new century, new teaching methods are under investigation. Problem-based learning is one such method believed to encourage the skills students need to succeed. It was hypothesized that problem-based learning can have impact on critical thinking ability of Iranian EFL students. The data was collected from Iranian students at Dorsa language institute located in Hashtgerd, Alborz province. To homogenize the participants, the researcher had a language proficiency test (PET) as pre-test to 71 EFL learners. Out of 71 students, 40 students were selected. They were divided into one experimental and one control group based on their scores. The researcher used independent sample T–Test to measure the statistical differences between the two groups. Before receiving instruction the students completed a critical thinking questionnaire. After sixteen sessions of problem-based instruction, the researcher administrated the post-tests to both experimental and control groups. Finally, the results of the analysis of the data revealed that doing problem-based activities enhanced critical thinking ability of the subjects. The findings of this study confirmed that participation in problem-based learning class had a significant effect on EFL learners‟ critical thinking ability.