A pH-stat MSMPR crystallizer is used to obtain the nucleation rate equation of calcite formed in the Na2CO3-CaCl2 reacting system. The experiments are conducted for seeded and unseeded runs. Effects of breakage and agglomeration on the crystal-size distribution are demonstrated. In a continuous operation, the agglomeration of nuclei followed by the breakage of agglomerates occurs at the transient state, while the agglomeration is significant at the steady state. Taking crystal agglomeration into account, the corrected nucleation rate and agglomeration rate are obtained simultaneously. The agglomeration rate is a function of relative supersaturation and magma density, and the corrected nucleation rate is approximately two- to fivefold of the apparent nucleation rate, which is the nucleation rate without considering the agglomeration effect. Discussed also is the formation of calcium carbonate polymorphism.