This research aimed to improve students' mathematical problem solving and mathematical disposition ability through Problem-based learning with Group Investigation Strategy in grade VIII C of SMP N 1 Kalibagor. The research subjects are 34 students of grade VIII C. The data collection methods are observation, test, questionnaire, and documentation. The research result indicates that the average scores of the students' mathematical problem solving skill are 58.27 in cycle I, 66.55 in cycle II, and 67.16 in cycle III. Moreover, the average scores of students' mathematical disposition skill are 68.40 in cycle I, 69.48 in cycle II, and 71.07 in cycle III. Therefore, it can be concluded that the implementation of Problem-based learning with Group Investigation Strategy improves the students' mathematical problem solving and mathematical disposition ability