We examined the crystallization of CaCO3 by changing the time of addition of the G1.5 and G3.5 poly(amidoamine) dendrimers to a calcium carbonate solution after incubation for several minutes (1, 3, 20, or 60 min) at 30degreesC. Based on the fact that the crystal phase obtained in the absence of the dendrimer was calcite, the dendrimers effectively stabilized the most unstable vaterite particles, even delaying the addition time of the dendrimer for 60 min. As the delayed addition time of the G1.5 dendrimer increased from 0 to 60 min, the particle size increased from 1.2+/-0.5 to 4.7+/-0.7 mum. As the generation number of the dendrimer increased from G1.5 to G3.5, the particle size of the spherical vaterite decreased under the same condition. The present results suggest that the anionic PAMAM dendrimers effectively modified the metastable vaterite surface and inhibited the further growth of vaterite particles by changing the addition time.