This research was classroom action research (CAR) which aim to increase ability of understanding mathematic concept class VIIB SMP Negeri 1 Bonai Darussalam with totaling 30 students. The study consisted of two cycles with the first cycle and consists of two meeting and daily test I and the second cycle and consists of two meeting and daily test II. The result of research showing existence of improvement understand concept of mathematics ability passing aplication learning model of inquiry. Those matter can be seen indicator ability of understanding mathematical concept was (1) student capable to express to repeat concept before action those were 9 studens (30%) and after action those were 15 students (53,33%), (2) student capable to differentiate of example and non example before action those were 12 students (40%) and after action those were 18 students (60%), (3) student capable aplication of conception before action those were 13 students and after action those were 22 students (72,17%). From those research can be concluded that applying model study of inquiry can improvde ability of understanding mathematic concept of students.