The object aim of this experimental study was to investigate the effect of dynamic geometry software (GeoGebra) on Grade-12 students’ mathematical thinking and mathematical achievement in analytic geometry. In the widespread of mathematical thinking aspects, six aspects: Generalization, Analytical thinking, Logical thinking, Abstract thinking, Problem solving and Representation thinking were included, while, mathematical achievement was simply procedural knowledge. A true experimental design (post-test equivalent group design) was used. And the four major objectives of this study were selected as: (i) to develop the mathematical thinking model of six aspects for analytic geometry; (ii) the criterion-test (post-test) under the constraint of this model; (iii) to find out the effect of GeoGebra aided instructions on grade-12 students’ mathematical thinking and mathematical achievement in the subject of analytic geometry; and (iv) to explore the effectiveness of GeoGebra aided instructions on high and low achievers students. Before experiment, the post-test was piloted; the item analysis along with reliability of the whole post-test was found. For experiment, about twenty-two lessons of GeoGebra, relevant to analytic geometry were designed in a well-constructed way. In comparison to the control group, the experimental group was taught through GeoGebra in the well-equipped computer lab. While, to assess the students’ responses, the criterion referenced test (posttest) was used. After experiment, the data were collected from the post-test and were analyzed through SPSS. The t-test, ANOVA and MANOVA tests were used to find out the significant difference between the various groups of experimental and control groups. xviii The main study findings showed that there were statistically significant differences between the mathematical thinking and mathematical achievement scores of the two groups. In the result, the experimental group developed significantly better in both mathematical thinking and mathematical achievement except in mathematical achievement for high achievers. In multiple comparison, in five aspects of mathematical thinking: Generalization, Analytical thinking, Logical thinking, Abstract thinking and Representation thinking out of six, the experimental group students performed significantly better. The only aspect problem solving, the mean score of the experimental group improved but with no statistical significance.