In this study, we investigated the accuracy of pupils´ self-assessment in two main school domainsmathematics and Czech language. The analysis explores whether pupils are able to evaluate adequately their own results in the didactic tests and then use some individual parameters to explain the level of selfassessment.The aim of the study was to analyze whether groups of pupils with different self-assessments of school tasks in the Czech language and mathematics (significant underestimation, adequate self-assessment, significant overestimation) differ in some of the cognitive skills studied.Our study questions were as follows: (1) Do pupils assess their achievements in particular school tasks accurately, or inaccurately? (2) Do pupils´ self-assessments differ in mathematics and language? (3) Do the pupil´s self-assessment correlate with individual parameters?The main tool used in the study was a didactic test on mathematics and a didactic test on the Czech language based on the Czech National Curricula Document and created by an expert team. In addition, Raven's Color Progressive Matrices (CPM), Similarities from the Wechsler Intelligence (WISC-SIM), and the Rey-Osterrieth Complex Figure (ROCF) were used. Considering the nature of the data, the non-parametric Kruskal-Wallis ANOVA was used.The present study is a part of the larger research project, involving 29 primary school classes, 657 pupils in total.Based on the data obtained, it can be concluded that the accuracy of pupils' self-assessments is low, while the accuracy of pupils' self-assessments in mathematics and Czech language differs (in mathematics there are more children with more accurate estimates and more pupils who underestimate themselves, in Czech language there are more pupils who overestimate their performance. Statistically significant differences were observed in the domains of Raven's Color Progressive Matrices and Rey-Osterrieth Figure, and in terms of the focus of each test, it could be concluded that there are significant differences between the groups in the domain of non-verbal reasoning skills and in the domain of analytical and organizational perceptual activity and memory. In the area of verbal intellectual abilities, there were no significant differences between the groups.