In this study, we proposed the learning activities using open community systems to increase critical thinking and cooperative work. We designed and implemented the Wiki system that is an opened community and general Web-Board system that is a closed community. Then, elementary students had the critical essays activities using these systems. The results of the study are as follows. First, the critical thinking activities were happened more actively at the Wiki system than at the Web-Board system. And we found that the critical thinking dispositions were improved at the Wiki system after students performed the critical writing activities. The critical thinking activities also occurred more actively in cooperative groups than in studying individual groups. In conclusion, when the students learn with the open community like the Wiki system than closed community, it is hoped that they would participate actively and their critical thinking would be improved better.