The effect of microwave on reactive crystallization is studied by measuring the induction time when nucleation is predominantly homogeneous and predominantly heterogeneous, respectively. Calcium sulfate is used as the working substance precipitated by mixing aqueous calcium chloride and sodium sulfate solutions. The experiments were carried with various microwave energy inputs. It is observed that the induction time significantly decreases with the increase of microwave energy input. The mechanism of the microwave effect on primary nucleation is analyzed. The main effect of microwave on primary nucleation is to increase the diffusion coefficient (D-AB). The surface tension (gamma(CL)) changes only slightly or remains constant in the microwave field. In addition, when nucleation is predominantly heterogeneous, the effect of microwave on contact angle (theta), geometric factor (f) is not significant. This suggests that diffusion coefficient acceleration is the main reason for the reduction of the induction time. It is also found that D-AB-D-0 has an exponential relationship with the energy input value of microwave. At the same energy input level, compared to homogeneous nucleation, the effect of microwave on diffusion coefficient was reduced by microscopic particles in the solution, when nucleation is predominantly heterogeneous. (C) 2017 Published by Elsevier B.V.