Background: Cooperative learning activities provide active participation of students leading to better learning. The literature suggests that cooperative learning activities need to be structured for a more effective and productive interaction. Purpose: This study aimed to test the differences among three instructional conditions in terms of science achievement. Sample: A total of 79 fifth-grade students, 42 males (53%) and 37 females (47%), participated in the study. Design and Methods: In the first condition, students answered the teacher's questions individually by raising hands. In the second condition, students discussed the answer in groups and came up with a single group answer. In this condition, the teacher provided only verbal directions to the groups without using any strategy or material. In the third condition, students used a "peer assessment form" before giving the group answer. A pre-/post-test experimental design was used. Multiple-choice and open-ended tests were used for data collection. One-way analysis of variance (ANOVA) was conducted to test the differences in the test scores between the three groups (individual answer, unstructured group answer and structured group answer). Results: Results showed that there were no significant differences among the three learning conditions in terms of their multiple-choice test scores. In terms of the open-ended test scores, students in the structured group answer condition scored significantly higher than the students in the individual answer condition. Conclusions: Structuring the group work through peer assessment helped to monitor the group discussion, provided a better learning compared to the individual answer condition, and helped students to participate in the activity equally.