The purpose of this study is to investigate the effects of using geometric functions approach on 9th grade students' motivation levels toward mathematics in functions unit. Participants of this study were 87 students who were ongoing in the first year of high school in Turkey. In this research, pretest and posttest control group quasiexperimental design was used. In the experimental group I, geometric functions approach and dynamic mathematics software which supported the mathematics teaching were used. In the experimental group II, dynamic mathematics software which supported the mathematics teaching was used. In the control group, traditional mathematics teaching method was used as a teaching method. The data of this study were collected by Students' Motivation toward Mathematics Learning questionnaire (SMTML). Before the experimental process, there was no statistically significant difference between students' motivation levels toward mathematics. After the experimental process, there was not a significant difference between experimental group I, experimental group II and control groups except for achievement goal dimension post-test scores. In achievement goal dimension there was a statistically significant difference between groups. The results of this study indicate that using geometric functions approach in the learning process of function concept has a significant effect on students' achievement goal motivation.