This study was conducted to help English teacher candidates maximize their efforts to develop critical thinking skills through an Extensive Reading (ER) program. There were forty research participants, from 18 to 21 years age range, in their fourth semester of undergraduate study in English Education Department. The main instrument to collect the data was a test in Critical Thinking, which was given before and after the program. The students were also asked to share their perception about the program after it ended by writing a reflection. During the ER program, the students practiced exercising their critical thinking skills in the post-ER activities which integrated all the four language skills. They needed to demonstrate their critical thinking as a required part of the assessment to provide evidence for their ER activities. Findings from the Critical Thinking test indicate improvement in the students' critical thinking skills after the program. They also had positive perceptions of the activities and assessment in the program, which they thought to have positive influence on their critical thinking. All these imply the potential of integrating the development of critical thinking skills in an ER program.