The purpose of this study was to know the influence of supervised enquiry instructional approach on mathematical problem-solving ability of Bulok Tanggamus Public Junior High School (SMP Negeri) 1. To compare the achievement of the students' mathematical problem solving ability by using supervised enquiry with the students using conventional instructional approaches. The type of the study was quasi-experimental with pretest – posttest-control group design. The population were students of eighth grade at SMP Negeri 1 Bulok Tanggamus. There were 119 students which consist of 4 classes, with 29 students from class VIII-A, 30 students from class VIII-B, 30 students from VIII-C, and 30 students from VIII-D. The samples in this study were students of class VIII-C as the experimental class and the students of class VIII-B as the conventional control class, which were selected by purposive sampling technique, with the consideration that the two classes have about the same level of ability. The instrument of this study was a 5 items test in essay form. It was designed to measure student's ability before and after the instructional process. The result were as follows: (1) the average increase in problem solving ability in the experimental class had reached 0.70, meanwhile the average increase in the control class was only 0,56. (2) The average increase in mathematical problem-solving ability of students who learned with supervised enquiry instruction was better than students who learned with conventional instruction. Thus the supervised enquiry instruction had more influence in the improvement of students' mathematical problem solving ability at SMP 1.