Crystallization of CaCO3 in the presence and the absence of half-generation poly(amidoamine) (Gn.5 PAMAM) dendrimers was carried out by a double jet method to prevent heterogeneous nucleation at glass walls. The solution was kept at 25degreesC under N-2 for 4 days with gentle stirring. We found that the crystallization of CaCO3 in the presence of the anionic dendrimers resulted in the formation of spherical vaterite crystals, whereas rhombohedral calcite crystals were formed in the absence of additives. As the generation number of the PAMAM dendrimer increased from G1.5 to G3.5, the particle size of vaterite was decreased from 5.5 +/- 1.1 to 2.3 +/- 0.7 mum under the constant concentration (0.26 mM) of -CCONa unit of the PAMAM dendrimers. With further increase in the generation number to G4.5, the particle size was not changed. The relationship between concentration of the dendrimer and the particle size in the same generation numbers of the PAMAM dendrimer was also studied under the same condition. As concentration of-COONa increased from 0.26 to 8.33 mM, the particle sizes of spherical vaterite were reduced from 5.5 +/-1.1 to 2.5 0.6 mum.