The aim of this study was to investigate the effect of using dynamic geometry software (GeoGebra) on the eighth-grade students' conceptual understanding and the retention of learning regarding linear equations and slope. In this study, a quasi-experimental design with pre-test, post-test and delayed post-test was employed. This study was conducted with 52 eighth-grade Turkish students (experimental group, n = 25; control group, n = 27). While GeoGebra software-supported instruction was carried out in the experimental group, textbook-based direct instruction was continued in the control group. Data were collected with the conceptual understanding test (CUT) which consists of 38 questions including open-ended, multiple choice and fill in the blanks. CUT was applied to the experimental and control groups as a pre-test and post-test at both the beginning and the end of the instruction, respectively. Seven weeks after the instruction, CUT was applied to both groups as a retention test. Data were analysed through SPSS 17.0 statistical software by using a t-test and ANCOVA test. It was indicated in the study results that GeoGebra software-supported instruction for eighth-grade students regarding linear equations and slope did significantly improve both their conceptual understanding and retention of learning in comparison to textbook-based direct instruction.