This study explored the effect of origami practice on size comparison strategies among young Japanese and American children. Forty-six Japanese children and 48 American children, 4 to 6 years old, received three different treatments. Specific treatment children folded origami paper triangles, superimposing the paper shapes to achieve congruence; general treatment children folded traditional origami forms; and control children engaged in conversation with an experimenter. In each task of the pretest and the posttest, children in all conditions were asked to judge the relative sizes of two figures. It was found that the girls under the specific treatment condition increased their strategy of superimposition significantly more than those under the control condition. In addition, the number of children changing their dominant strategy to superimposition or adjustment by two dimensions after the specific or general treatment sessions was significantly larger than that of children changing their strategy to a less effective strategy. The implications of these results for education are discussed.