Based upon self-concept and self-determination theory, the present study aimed to examine the measurement separability of young children's math self-concept (MSC) and six specific types of motivation, namely, intrinsic motivation (INTR), integrated motivation (INTEG), identified motivation (IDENT), introjected motivation, extrinsic motivation (EXTR), and amotivation (AMOT). Moreover, the relations between these constructs and math grades (MGs) were explored. Participants were 397 elementary school children from Grade 4. Results provided support for the measurement separability of these six types of motivation, but not for a strong differentiation between these specific types of motivation and MSC. In addition, there were significantly positive relations of MSC, INTR, INTEG, IDENT, and MG, while significantly negative relations of MSC, EXTR, and AMOT appeared. Support for these relations was also evidenced by further regression analyses irrespective of the direction of the variables. The strongest relations were found for MSC and MGs.