Recent empirical research has demonstrated the importance of mastery motivation in child development. Therefore, it is essential to have valid and reliable instruments to measure this variable. The Dimensions of Mastery Questionnaires (DMQ 18) was validated in English, Hungarian, Chinese and Spanish. In this article, we evaluate the psychometric properties of Romanian and Russian versions of DMQ 18 that were piloted simultaneously. The study sample consisted of 315 fifth-grade students studying in Russian language schools or Romanian language schools. Reliability of these two versions was demonstrated by good internal consistency. Factor analysis that fits well the theoretical dimensions provided evidence of construct validity. Moreover, this publication presents a comparative ethnic study of Russian- and Romanian-speaking school-aged students from the Republic of Moldova and highlights ethnic differences in mastery motivation as perceived by students.