Based on the different nucleation ability, different contents and proportions of the mixed nucleating agents were melt compounded with pure isotactic polypropylene resins employed in this work, selective beta nucleating agent was a compound of pimelic acid and calcium stearate, the a nucleating agent was a type of dibenzylidene sorbitol derivative. The competition attributed to the different type of interactions between a nucleating agent and isotactic polypropylene molecular chains during crystallization was investigated in this research. For a low content of the mixed nucleating agents used, a nucleation dominated the crystallization behavior of isotactic polypropylene, while with an increase of the content of the mixed nucleating agents, the situation was reversed and beta nucleation became dominant. In the beta dominated region, both of the molecular chain and segmental motions were various with different nucleating agent proportion. Thus, the content and proportion of the components of the nucleating agents were the critical factors in the competition during crystallization.