The purpose of this study was to investigate the effect of Augmented Reality (AR) assisted learning environment on students' learning performance. A total of 30 students were invited to learn the topic of geometry in different instructional settings. One group of students learned geometry with only paper information, another with paper information and real bricks, and the other with an Augmented Reality system (AR App). To examine the learning performance, students were tested with the Mathematics Achievement Test (MAT) before and after the geometry learning. The result showed that there was significant difference in the learning achievement (post-test) among the three study groups. Students of AR group obtained the highest scores in MAT.