The present study investigated to what extent teachers' beliefs about children's achievement contribute to the development of children's math interest. In addition, the extent to which other possible predictors, such as performance in math, gender, and race/ethnicity would contribute to the development of children's math interest was examined. Three cohorts of children (N = 849) and their teachers participated in the study starting from kindergarten through their sixth grade. The results showed that especially teachers' beliefs about children's effort and potential performance positively predicted children's interest in math across the primary school years, whereas teachers' beliefs about children's math ability predicted children's math interest only at the beginning of primary school. Further, all the models were similar for boys and girls and for children in different cohort groups.