The purpose of this study was to investigate the relationship between students'achievement level and the validity of students' self-assessments in the coursesocial studies. The students were divided into two achievement groups,low-achiever and high-achiever. Low achievers were students with the grade F-Din the subject of social science and high achievers were students with the gradeC-A in the subject of social science. The students had to do a test which they thenhad to self-assess. The questions on the test were divided into three abilities;factual knowledge, understanding and skill. The students did threeself-assessments, one for each ability. The study found that there is a positivecorrelation between the students' level of achievement and the validity of theself-assessments. This was in line with previous research (Yao-Ting, 2010). Thelow achieving students overestimated an average of 0.7 grade points compared tothe high achievers who overestimated an average of 0.3. The study saw a certaindifference in the validity of self-assessments depending on ability, factualknowledge was the ability that the students overestimated themselves most.