Epistemological beliefs are assumptions and beliefs that people have about knowing and knowledge. Education setting is essential factor in epistemological beliefs development. By using various teaching methods, teachers may create specific epistemic climate. In this research based on quasi- experimental design fifth grade students at two science classes were compared. In one school two classes were homogenized based on students IQ and academic achievement. Experimental group received problem- based learning whereas in control group regular teaching method which emphasized on simple grouping of students for accomplishing class tasks was used.The study was conducted during a four- week unit about environment pollution, which was the first of the four science units studied in the fifth grade. Epistemological beliefs were measured along four dimensions (Source, Certainty, Development and Justification) with a 26-item questionnaire which was developed by Elder. In addition, the quality of students’ worksheets were analyzed and compared. Findings indicated there was no significant difference (t= 1/131, sig=0/263) between two groups in epistemological beliefs.But regarding the quality of worksheets, experimental group performance in organization, completion of required section, neatness… was higher than students in control group. It is concluded, to change epistemological beliefs spending more time than 4 weeks is necessary; because it is a developmental variable.But higher performance of experimental group students in activity sheets may be related to PBL essence. It seems applying PBL in class brings about more motivation and purposeful cooperation among students.
