A teachable-agent arithmetic game is presented and evaluated in terms of student performance, attitude and self-efficacy. An experimental pre-post study design was used, enrolling 153 3 rd and 5 th grade students in Sweden. The playing group showed significantly larger gains in math performance and self-efficacy beliefs, but not in general attitude towards math, compared to control groups. The contributions in relation to previous work include a novel educational game being evaluated, and an emphasis on self-efficacy in the study as a strong predictor of math achievements.