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