Spatial ability has been valued as a talent domain and important skill in mathematics education because it enhanced an intuitive view and an understanding in many areas of mathematic. In addition, spatial ability highly correlates with mathematics achievement, indicating its crucial role in success in mathematics education. Some researchers founded gender differences in mathematics and spatial ability, and indicated that spatial ability served as a mediator of gender difference in mathematics. This study explored the spatial ability of 349 Korean middle school students (Grade 7-9), and investigated the association among students`` spatial ability and their mathematics achievement, gender, and grade. The result of this study shows that spatial ability correlates positively with mathematics achievement. While gender difference did not exist in mathematics, significant gender difference existed in spatial ability favoring male students.