The Blinder-Oaxaca approach has been used to decompose the gender gap in mathematics test scores in private and public primary schools in Kenya. That is, the mathematics production functions are estimated separately for boys and girls, the difference between the boys' and girls' average scores is estimated and then decomposed into three components. That is, gap due to differences in resources between boys and girls, gap resulting from difference in utilization of educational resources and the gap due to unobservable factors. The results show that boys outperform girls in mathematics in both school types. In both private and public primary schools, boys take better advantage of the resources compared to the girls hence girls' scores can be improved by better utilization of the resources. In addition, the boys' scores in public primary schools can be improved by increasing the educational resources for boys. This indicates that resource based policies aimed at improving the girls' mathematics scores may not necessarily close the gender test score gap. Further, such policies may reverse the gender gap and disadvantage the boys. There should be focus on factors that improve utilization of resources by girls in order to improve their scores.