Human must have the ability to understand, because understanding can prevent people from misunderstandings and conflicts. Similarly, public junior high school (PJHS) students need to have mathematical understanding ability (MUA) for a reason, that is, MUA is an important part in problem solving. In fact, MUA of PJHS students was still low. This research was conducted to contribute in improving students’ MUA. There were 158 students engaged in the experiment classroom as well as in the conventional one taken from PJHS 1, 2, and 4 from district of Deli Serdang, PJHS 17 and 22 from Medan City, Indonesia. Joyful problem based learning (JPBL) approach was applied to attain the purpose of the research. The study used essay-test to measure students’ MUA. The score obtained was then analyzed by t-independent test, while student performance in solving MUA problems was described descriptively. Results of the research: (1) Students MUA’ score was higher in the experiment classroom than in the conventional one. (2) The improvement of students MUA in the experiment classroom belongs to medium category. (3) The students’ performance in MUA was better in the JPBL classroom than it was in the conventional one. Some students faced difficulties both in explaining the solution and in giving example of a mathematical concept. Overall, the students’ performance was best at the aspect of presenting problem in mathematics equation. Based on the findings, the study suggests teachers to give reinforcement in both aspect where students lacked of by, for example, encouraging them to solve a variety of problems which eliciting the aspect of explaining and giving examples.