The crystallization in isotactic polypropylene (iPP) melt inclusions, encapsulated by transcrystallinity and spherulites, was investigated by polarized light microscope. Generally, owing to the proceeding of crystallization in melt inclusions, a negative pressure will be built up. Up to a critical value, the negative pressure is released by the formation of cavitation in melt. Unexpectedly, beta-crystals with different morphologies emerge at different locations in the melt inclusions. For low molecular weight iPP, only smaller beta-crystals around bubbles are discernable. Turn to higher molecular weight iPP, apart from the smaller beta-crystals, fan-shaped ones can be developed under suitable conditions (e.g., lower than 131 A degrees C) at the growth front of spherulites and transcrystallinity. Considering the characteristics of molecular weight and morphology, it is proposed that beta-crystals are induced by different mechanisms. That is, smaller beta-crystals are resulted from the stress caused by the appearance of bubbles while the fan-shaped ones are derived due to the temperature gradient during the release of negative pressure.