This current study examined human children's and adults' automatic processing of proportion using a Stroop-like paradigm. Preschool children and university students compared the areas of two sectors that varied not only in absolute areas but also in the proportions they occupied in their original rounds. A congruity effect was found in both age groups. The dimension of proportion interfered with adults' and children's area comparison greatly and comparatively. These findings strongly suggest that preschool children automatically represent proportion and provide evidence that this representation is intuitive and independent of formal school instruction.