The fracture mechanism of crazing in polypropylene (PP), the molecular weight of which was controlled by several different methods, was examined. The craze strength was improved with increasing number-average molecular weight in all specimens regardless of the weight-average molecular weight. Examination of the number of tie molecules between neighbouring lamellae suggested that craze fracture was caused by the failure of tie molecules. It may be expected from these results that PP of high fluidity can be produced without a drop in strength if the weight-average molecular weight approaches the number-average molecular weight under the retention of the number-average molecular weight.