Conceptual understanding is an aptitute to comprehend concepts or methods that have been explained not only memorized and students are competent to draw a conclusions from a given problem. This research aims to insvestigate the effect of using Geogebra software on students' understanding of mathematical concepts in the material of side room solid. This research is a quantitative study with a true experimental research design, spesially posttest-only control design. The paryicipants of this study were students of class VIII 1, VIII2, VIII3, and VIII4 SMP Negeri 02 Sekayu consisting of 121 students. The sample in this study was taken using purposive sampling technique. Based on this technique, 2 classes were obtained, namely the experimental class and the control class. Data collection techniques using tests in the form of descriptions with a total of 5 questions. Data analysis in this study used t-test data analysis. From the results of the t-test calculation obtained tt = 1.69 < to = 12.00. This means that there is an effect of using GeoGebra on geomtery material on the understanding of mathematical concepts of students in class VIII SMP Negeri 02 Sekayu. Keywords: Geogebra, Mathematical Concept Understanding Ability, Geometry.