The current study examines reciprocal influences of homework purposes (approval-seeking, self-regulatory, and academic) and math achievement, using data from 1365 students in grade 8 at two measurement points. Results indicated there were positive reciprocal influences of (a) academic purpose and achievement, and (b) academic purpose and self-regulatory purpose. Results further revealed that prior achievement had a positive effect on later self-regulatory purpose. Meanwhile, prior approval-seeking purpose had a negative effect on later achievement. Taken together, the present study advance extant research, by differentiating three types of homework purposes using models of reciprocal influences, and by showing their respective relationships with math achievement over time.