The crystallization conditions dependence of polymorphs composition in beta nucleated propylene/ethylene copolymers (PPR) and propylene homopolymers (PPH) were comparatively investigated via wide angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) measurements. It is interesting to note that the amount of beta form as a function of crystallization conditions presents an opposite trend for the beta nucleated PPR and the beta nucleated PPH under the conditions we investigated. For the beta nucleated copolymers, the content of beta form shows also an opposite tendency with that of gamma form with the change of crystallization conditions. The formation of gamma form is preferred under lower cooling rates or higher isothermal crystallization temperatures, whereas the amount of beta form increased with increasing the cooling rates or decreasing the isothermal temperatures. This opposite tendency could be interpreted in terms of the competition between the beta nucleation ability of beta nucleating agent and the gamma nucleation action of the comonomer defects. The existing comonomer defects that favor the formation of gamma form may suppress the nucleation ability of beta nucleating agent. A higher proportion of beta form in PPR containing a beta nucleating agent could be achieved under faster cooling rates or lower crystallization temperatures. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 3247-3254, 2010