The aims of this research are to decide the effectiveness and the comparison of the effectiveness of open-ended and problem solving approach toward matter of flat side construct lesson viewed from achivement of reasoning ability, problem solving and mathematics communication. This study was a quasi experimental study using the pretest-posttest nonequivalent group design. The research population covered the entire VIII class students' of SMP Negeri 1 Pandak, Bantul, Yogyakarta. From the population, classes of VIII F and VIII G were selected randomly as the research sample. To test the effectiveness of open-ended and problem solving approach, the one sample t-test was carried out. Then, to test the more effectiveness of the open-ended approach than the problem solving approach, the MANOVA was carried out and then continued by the t-Bonferroni test. The results of the study show that open-ended and problem solving approaches are effective and open-ended approach is more effective than problem solving approach in matter of flat side construct' viewed from achivement of reasoning ability, problem solving, and mathematics communication in junior high school.