Traditional studies of crystallization kinetics are often limited to idealized conditions where the temperatures or the cooling rates are constant. In real manufacturing processes, however, the external conditions change continuously, which make the kinetics of crystallization dependent on instantaneous conditions, especially on changing cooling rate. To obtain the crystallization information in manufacturing processes, lots of mathematical models for the non-isothermal crystallization kinetics are raised. But most of them concentrate on constant cooling rates melts crystallization behavior and pay little attention to the condition of varying cooling rates, which is more close to actual processing conditions. Based on the thermodynamics theory of crystallization, I.J. RAO and K.R. RAJAGOPAL derived a general specific model for quiescent crystallization (it is simplified as RAO model below). In order to verify the RAO model’s simulation effect on changing cooling rates crystallization, the constant cooling rates and varying cooling rates melts crystallization of isotactic polypropylene, high density polyethylene and nylon 6 were all investigated using the DSC technique. The results showed that the model predictions and experimental results were in good agreement.