The aim of this research was to examine the effect of GeoGebra-aided REACT strategy on the understanding of geometry concepts and to investigate the interaction between learning strategy and spatial ability on the understanding of geometry concepts. This research involved 60 students of Grade 8 at a private Junior High School in Borong, province of East Nusa Tenggara, Indonesia. The quasi-experimental research with control class without pre-test was done. Prior to the experiment, spatial ability test was conducted to group students into high and low spatial ability. The research data were obtained from a test consisting of five questions to measure students' understanding of geometry concepts. The data were then analyzed using two-way ANOVA. The results of the research show that 1) the GeoGebra-aided REACT strategy leads to a higher achievement of the students on the understanding of geometry concepts if compared to student in conventional groups (expository), and 2) there was no interaction effect between the learning strategy and spatial ability on students' understanding of geometry concepts. The research recommended that GeoGebra-aided REACT strategy can be used in mathematics teaching in Junior High School to improve students' conceptual understanding of geometry concepts.