The iPP sheet is extruded through a slit die, in which the extruding rate is set small enough to eliminate the shear stress as much as possible. Simultaneously, the extruded sheets were stretched with different stretching ratios (SR). With SR increasing from 1 to 6.67, fraction of beta-crystal increases dramatically from 1.5 to 13%, which indicates that SR is favorable to the occurrence of beta-crystal. However, as SR further increases, it gradually decreases to 0, which reveals that SR can steadily restrain the formation of beta-crystal. As the thickness of the sheets decreases with the increase of SR, higher cooling rate occurs for the sheets extruded at larger SR. When SR is in the range of 1-6.67, the cooling rate might probably fix the oriented chains and provide a proper thermal environment for subsequent growth of beta-crystal, thereby, fraction of beta-crystal increases. Growth of beta-crystal could be restricted by higher cooling rate, which leads to the gradual decrease of beta-crystal fraction with the further increase of SR. POLYM. ENG. SCI., 48:2454-2458, 2008. (C) 2008 Society of Plastics Engineers