The purpose of this study was to determine the effectiveness of computer-assisted instruction (CAI) versus traditional lecture-type instruction on triangles. Two quasi experiments were conducted in six 6th grade classes with a total of 108 students respectively. The students in the control groups were taught the concepts of triangles in their original classes, while the students in experimental groups were instructed in a computer lab. Experimental group students utilized Interactive Middle School Math Bundle, which is an interactive Webpage-typed tutorial. The tutorial, featuring descriptions, sound, animation and self-examination, allowed students to navigate and self-explore themselves. Independent-t was used to analyze the data. The analysis revealed that there was no statistically significant difference between the students' achievement in the control and experimental groups. The result implies that teachers could use computer-assisted instruction software only as a supplemental tool. Further research is recommended to examine effectiveness of computer-assisted instruction with an extended time span. (Contains 2 tables.)