The purpose of this study was to investigate the effectiveness of Project Based Learning in Geometry on Grade Seven Student' achievement and their geometry thinking. To achieve this purpose the following two questions were answered: 1- What is the effectiveness of Project Based Learning on Grade Seven Student' achievement? 2- What is the effectiveness of Project Based Learning on Grade Seven Student geometry thinking? The sample of the study was consisted of 58, grade seven students, of Marfa Daris School (5-10), which located in Willayat Nizwa, in AL-Dakelia Region. The students were distributed equally into two groups each group consisted of 29 students; the experimental group, which was taught through the proposed strategy, and the control group was taught in the ordinary way( the normal practices and instructional methods of the teacher). The researcher selected one math units of Grade 7 math book, for the academic year 2014-2015; Area and Size. The study lasted for 4 weeks, 7 periods of 45 minutes per week. At the end of the study the researcher applied the achievement test and the geometry thinking test on both groups. The means and standard deviation of the two groups were calculated, t-test was used to verify the significance of difference between the mean scores of the study groups. It was shown that: 1. There was a statistically significant difference at (a < 0.05) in the achievement test in favor of the experimental group. 2. There was a statically significant difference at (a < 0.05) in the geometry thinking test in it's three levels (Visualization, Analysis, Informal Deduction) in favor of the experimental group. Based on these results, the researcher recommended to conduct more studies about using this strategy in all aspects of teaching and learning, and to promote using Project-Based Learning in teaching and learning.