The research is a classroom action research (CAR) conducted due to problems in which most of Mathematics lessons give emphasis only on memorization, students' low motivation in proving the truth of a problem, and students' low hard work attitude. In the 21st century, one of the requirements for students is that they have to be able to think critically and to have hard work attitude. This research implements Problem-based learning in improving grade VIII A students' critical thinking ability and hard work attitude in SMPN 1 Kalibagor. The research actions are conducted within three cycles consisting of two meetings for each cycle. The research result indicates that the average evaluation score of the students' mathematical critical thinking ability improves from 65.18 in cycle I to 71.61 in cycle II to 75.65 in cycle III. Moreover, it is also found that the students' hard work attitude improves from cycle I to cycle III, which is proven from the scores of the observation sheets and the hard work questionnaire. Thus, it can be concluded that the implementation of Problem-based learning improves students' mathematical critical thinking ability and hard work attitude in VIII A grade of SMP N 1 Kalibagor.