The impact of two types of written feedback (process-oriented, grade-oriented) on changes in mathematics achievement, interest and self-evaluation was compared -- with a particular focus on the mediating role of feedback's perceived usefulness. Participants, 146 ninth graders (aged 14 to 17?years), were assigned to either a process-oriented or a grade-oriented experimental feedback condition. They worked on mathematics tests, received feedback on their test results and completed surveys measuring feedback's perceived usefulness, interest and self-evaluation. Results of path analysis showed that process-oriented feedback was perceived as more useful than grade-oriented feedback and that feedback's perceived usefulness had a positive effect on changes in achievement and interest. Consistent with this, process-oriented feedback had a greater positive indirect effect than grade-oriented feedback on changes in mathematics achievement and interest via its perceived usefulness. There were no such effects on changes in self-evaluation. Potential explanations for these findings, educational implications and possible directions for future research are discussed.