This study aimed at investigating Van Hiele's geometric thinking levels at grades seven, eight, and nine of Basic Education in Oman. Three geometric thinking levels (visualization, analysis, informal deduction) and their development at each of the three classes were studied. Finally, the effect of gender on students' performance at geometric thinking levels was also studied. Sample of the study consisted of (347) male students and (367) female students, constituting (714) students. These students were selected from (8) schools that were randomly chosen from (33) Basic Education government schools at Muscat region during the academic year 2007/2008. Data of the study was collected by the means of a written test that has been designed to measure geometric thinking levels as described by Van Hiele's theory. This study found that 9.3%, 13.2% and 6% of students in grades seven, eight, and nine respectively did not pass any of the geometric thinking levels, and (89.5 %) Students from seventh grade, (81.2%) of eighth grade students, and (91.5%) of students in ninth grade, were classified in one level of the geometrical thinking levels, according to the expected order of geometrical thinking levels (visualization, analysis, informal-deduction) of Van Hiele theory. The results showed there were significant statistical differences at (a < 0.05) between the averages of students in three classes (seventh, eighth , ninth) in the three levels of thinking (visualization - analysis - in formal -deduction), which proves the existence of the development in geometrical thinking levels in grades (seventh, eighth , ninth) in three levels (visualization - analysis - in formal -deduction), this development became clear in the ninth grade for three levels (visualization - analysis - in formal -deduction). The researcher has found out that sex does not affect the performance of student in geometrical thinking levels test. It has been recommended to take care of introducing the definition of modern theories of teaches geometry to the teachers, and the teachers concerns by selecting the appropriate methods and techniques for teaching geometric units to suit the level of students for the development of geometric concepts, review of the geometric units for the eighth grade activities include deepening geometric concepts to students.