The purpose of the study was to investigate the effects of the geoboard activities in understanding the Pythagorean Theorem. Five groups of middle school students were involved in the study. The research questions of the study were followings: 1)What are the differences in understanding and attitudes among students those who revealed the various levels of achievement when geoboards were introduced in learning the Pythagorean Theorem. 2)What was the effect of the geoboard activity in introducing the Pythagorean Theorem and solving relevant problems? 3)What would be the impression of geoboard activity for those who already knew the Pythagorean Theorem? 4)What would be the effects of interaction in geoboard activities? 5)What was the effect of the geoboard activity in recovering the Pythagorean Theorem, and applying the theorem. The result of the study revealed the positive effects of geoboard activities throughout the research questions although there were differences among various levels of students and groups.