This study investigates the effects of dynamic geometry based computer instruction on eighth grade students' achievement in geometry and their attitudes toward geometry and technology compared to traditional instruction. Central to the study was a controlled experiment, which contained experimental and control groups both instructed by the same teacher. 36 eight grade students from a middle school located in a rural area in Turkey participated in this experiment. None of the students had past experience in using computers especially for the purpose of learning mathematics. The results revealed that dynamic geometry based computer instruction had a significant effect on students' achievement in geometry compared to traditional instruction. Furthermore dynamic geometry based instruction had a significant effect on students' attitudes toward geometry and technology compared to traditional instruction. A correlation analysis showed a positive correlation between the achievement levels of the students and their attitudes towards geometry and technology.