This study aimed to investigate the effects of computer aided instructional applications on mathematics teacher candidates' geometry success and determine their perceptions on learning and teaching geometry supported by software. The study is a parallel-simultaneous mixed method research due to use of qualitative and quantitative data together. Within the scope of the research conducted, 30 teacher candidates attending Department of Elementary Mathematics Education at Eskişehir Osmangazi University Faculty of Education were selected as the participants of the study. The results of the research show that Geometer's Sketchpad (GSP) course applications ensure teacher candidates to increase their geometry success with also respect to t-test results. It is understood teacher candidates find the software activities and spreadsheets sufficient, especially in demonstration of proofs. The computer aided -student centered instructional method used in this research takes positive reaction by the candidates and software language, tutorial time and the physical characteristics of schools take attention as the critique aspects of applying software tutorials. The participants also noted that the way of development of software activities and spreadsheets affects the way of choosing suitable approach for the instruction.Keywords: Computer aided instruction, mixed-model inquiry, geometry instruction. SummaryLearning geometry is an important part of mathematical ability. Computer algebra systems and geometry softwares present quite advantageous samples for manipulating geometric and numeric realities. Among these, Geometers Sketchpad is an example for tools manipulating geometric objects. For applying such tools for instruction, it is important to determine how eager teacher candidates are for using this technology in their lessons and, to what extent they improve their achievement scores by using such tool in order to determine their level. In this regard, this study aimed to determine the outcomes of software use within the context of geometry instruction. Participants consisted of 30 fourthyear elementary mathematics teacher candidates attending Eskişehir Osmangazi University, Faculty Of Education. A parallel-simultaneous mixed methodology was conducted to collect both qualitative and quantitative data at the same time. The experimental part of the study is designed by pretest-posttest experimental design without a control group and one researcher used a diary during all nine-week research process. After the intervention, a semi-constructed interview was conducted to collect additional qualitative data.