This study aimed to investigate the effect of using self assessment in the achievement and reflective thinking in social studies. The researcher used quasi-experimental design. The sample consisted of (63) students of tenth grade in the basic education schools in the Northern Sharqiya. The sample was divide into two equal groups: an experimental group (32 students), and a control group (31 students). To achieve the objectives of the study, the researcher used two instruments to collect data: an achievement test and a reflective thinking test to measure the effect of self- assessment on achievement and reflective thinking. To verify the validity of the test, it was presented to a group of arbitrators. The stability was calculated using the alpha coefficient of Cronbach's alpha which was (0.779) for the achievement test and (0.748) for the reflective thinking test.The results of the study showed statistically significant differences at the level of (α ≤ 0.05) between the mean of the students' scores of the experimental group and controlled group in the achievement post-test and reflective thinking post-test in the favor of the experimental group.The study recommended that students should be trained to use methods of self-assessment to judge their performance, involve them in setting the criteria upon which they are evaluated, and introduce self- assessment in addition to the approved official assessment in social studies.