As a fundamental study of zeolite crystallization from aluminosilicate solutions, the critical radii of nuclei of zeolite A and the amorphous phase were estimated based on the nucleation rates determined in our previous studies and the classical theory of homogeneous nucleation. The calculated critical radii of nucleus of zeolite A crystals were 0.4-0.8 nm under the conditions examined, and its diameter is found to be almost equal to the half of the edge length of a unit cell of zeolite A crystals. Meanwhile the radii of the critical nucleus of the amorphous phase were 0.8-1.6 nm with the surface energy of 12-23 mJ(.)m(-2). These calculated results imply the following phase transformation processes. For the case of zeolite A nucleation, several growth units assemble together and result in a formation of a zeolite A nucleus. In the case of the amorphous phase, a greater number of growth units are needed for nucleation and the induction time of the nucleation of the amorphous phase is shorter than that of zeolite A probably due to the random structures of the amorphous phase.