Accurate assessment of preschool children is challenging, with studies of dynamic measures of early math performance missing from the field. Therefore, curriculum-based measures that inform planning and instructional decisions are of particular interest. The purpose of this research was to evaluate a constructivist mathematics curriculum-based measure (MCBM) for assessing number sense in preschool children. The hypothesis was that six measurable variables¿quantification level, counting skills, set comparison, numeral recognition, combining sets, and patterning¿would make significant contributions to the latent construct of number sense. The measurement tool was an interactive game consisting of teddy bear counters and a series of cards in a standardized order, played between assessor and a child. The sample consisted of 108 children, ages 3¿5 years. Confirmatory factor analysis, used to evaluate the hypothesized model, indicated a good model fit: ¿2(3,N=108)=4.246, p=.236; NFI=.990; CFI=.997; and RMSEA=.062. Standardized path coefficients ranged from .71 to .88. The results suggest that the MCBM was a valid and reliable measure of number sense for this sample.