Introduction: Peer evaluation is a method in which students evaluate their classmates. Experts recommend using this method. This study was conducted with the aim of evaluating the effect of cooperative assessment method on nursing students' learning in the operation room. Materials and Methods: In this quasi-experimental study, 30 operation room internship students were randomly classified into two groups of sample (assessment by the team member and instructor) and control (assessment by the instructor). Evaluation was performed by the check list and its validity was confirmed by instructors. Finally, data were compared between the two groups using t test and SPSS 13 software. Results: Comparing the mean scores resulting from classmate assessment and that of the instructor showed no significant difference (t= 0.43, p <0.08, 18.50±1.25 vs. 19±0.75). The group assessed by the instructor and the team member (18.5±1.25) has acquired higher scores compared with the control group (18.25±1.50). However, this finding was not statistically significant. Conclusion: Although it seems that the classmate assessment has no significant effect on the final score, considering the similarity between classmate’s assessment scores and those of the instructor, it seems that the student’s assessment of his team member can be a reliable measure in the evaluation of students.