The goal of this research was to detect the connection between mathematics and language self-concept of elementary school aged children. The sample was formed of 243 elementary school students across grades 3-8. The research was done to examine the development of students´ mathematics and native language self-concept as well as their achievements in relation to gender and age. That is why the mathematics and native language self-concept (including PISA education clames) have been used in this research along with general questions. The results have shown that there are no gender differences in mathematics achievements, but both genders show declining achievements in relation to age. Students with higher self-concept in mathematics achieve better results in Mathematics. Females tend to have higher self-competence perception in languages than males, so they can achieve better results in Croatian language. Higher self-concepts in languages and the arts produce high levels of achievement in languages. Both, male and female students have higher self-competence perceptions of their language skills, interest for native language learning and its usefulness than that of mathematical skills, interest for Mathematics and its usefulness.