The aim of educational system which changes according to the enhancements in educational technology is to make students more active in the learning process, as well as make them as people who have skills such as critical thinking, problem solving etc. Critical thinking is a thinking skill which consists of mental processes of discernment, analysis and evaluation. Blended Learning is learning which combines online and face-to-face approaches. By means of online discussion forms, teachers can monitor the class discussions. These discussions help teachers identify topics that need clarification or that have captured the interest of students, and they can use the insights gained by them to structure class time. Also, if teacher use the Web discussion area to address some of the more straightforward student questions, they can make better use of class time. In this sense, the purpose of this study is to examine the effects of blended learning environment which supports the course management system on the critical thinking skills of students. The model of the study is pre-test, post-test single group model. There are 44 Students who attending the course of Design And Use Of Instructional Material in the Department of Computer and Instructional Technology Education of Ankara University in third grade who constitute the study group. In this study, Watson-Glaser Critical Thinking Appraisal Test, which consists of 100 items, was used to collect data. This test is composed of a series of test exercises in which the application of the important abilities in the critical thinking is involved. This test includes five sub-tests which are listed as; Inference, Recognition of Assumption, Deduction, Interpretation, Evaluation of Arguments. The test was developed in 1964 by Googwin Watson ve Edward M. Glaser. The test was translated into Turkish by Assoc. Prof. Dr. Nükhet Çıkrıkçı – Demirtaşlı in 1996. Analysis results indicated that the range of internal consistency of the subscales are interval from .20 to .47 and the total correlation coefficient is .63. The course consists of 5 weeks throughout the semester. At the beginning of the semester, student’ WGCTA scores were obtained. In the environment of blended learning, the course is supported to chat rooms and forums. After 5 weeks, students were asked to do the test again and the critical skills of the students were examined. The data was analyzed through paired sample t-test to compare the results of pre-test post-test scores, as well as descriptive statistics. There was no significant difference between pre-tets and post-test results.