This paper reports a study which investigated how elementary school students contributed differently to accomplish computer mediated collaborative tasks with different difficulty levels. One hundred and forty-five sixth grade students participated in this study. Three experimental activities were arranged. Each activity required the students in three-member groups to perform a collaborative task. The first activity required a hard task, the second one required an easy task, and the third one required a medium task. A collaborative learning system was adopted to support the activities. Students' textual discussions showed that students in accomplishing the easy collaborative task exhibited less desirable interaction with their partners.