The objective of this study is to examine the impact of instructing quadratic functions with the use of GeoGebra software and guided discovery worksheets on students' achievement and level of reaching acquisitions. This study utilized a pretest-posttest control group experimental design. It was conducted in the 2017-2018 academic year with 62 tenth grade students of a high school in the center of Balikesir, Turkey. The study data was collected using quadratic functions achievement test. The quantitative data collected in the study were analyzed using independent samples t-test. The findings of the study indicated that the instruction exercised with the use of GeoGebra software and guided discovery worksheets was effective in increasing achievement and the level of the capability to reach acquisitions compared to the control group; the findings also indicated that the instruction created a significant difference. This study also found that the students in the control group students focused on algebraic operations and formulas, mainly memorized the formulas, and were unable to build the relation between graphics and the algebraic expression of functions. On the other hand, the students in the experimental group were better in building the relation between algebraic expressions and graphics, and more successful in interpreting graphics.