The purpose of this study was to examine the limitations of traditional explanations for development and sex differences in mathematics, and to suggest the cognitive developmetal approach as an alternative. In this study, the characteristics of Miller's model as a general framwork for mathematical development were inquired and the possibility that Miller's model is an explanatory model for sex difference in mathematics was examined. The result indicates that all mathematics problems, including geometry and algebra problems can be divided by conceptual, algorithmic and symbolic competence as Miller suggeted. The cognitive developmental approach will help contribute to the decrease in the gender unbalance in mathemetics.