Compounds of isotactic polypropylene (iPP) and beta-nucleating agent were used to investigate the relationship between the development of beta phase and molecular weight in iPP under quiescent crystallization conditions by using wide angle X-ray diffraction and differential scanning calorimetry techniques. In all cases, the dependency of the formation of beta phase in iPP on molecular weight of iPP at a defined crystallization temperature range was found. The iPP with high molecular weight possessed a wide range of crystallization temperature in inducing rich beta phase. However, poor or even no beta phase was obtained for the samples with low molecular weight in the same range. In addition, an upper critical crystallization temperature of producing dominant beta phase was found at 125 degrees C. Beyond this temperature, a phenomenon of prevailing alpha phase became obvious. (C) 2014 Wiley Periodicals, Inc.