The purpose of this research is to find out whether there are: (1) the effect of discovery learning on mathematical understanding concepts ability, (2) the interaction between instruction and the prior mathematical ability towards mathematical understanding concepts ability. This research is quasi-experimental research. The population is all seventh grade students of Bina Bersaudara in Medan. Sample was taken by using total sampling technique so that the sample of the research was the seventh grade students then using randomly selected the experimental class was treated with discovery learning and control class with conventional learning. The instrument of the research are the mathematical understanding concepts ability test. Data were analyzed by using two way anova. Based on the results of the analysis obtained the results of the study are: (1) there is the effect of discovery learning model on the mathematical understanding concept ability, (2) there is no interaction between instruction and students prior mathematical ability towards mathematical understanding concepts ability.