A method is described for obtaining a quantitative measure of crystalline hydrocarbon present in stretched rubber samples by comparing the intensities of the diffraction spots and the amorphous halo appearing in the x‐ray diagrams. This method has been applied in studying the crystallinity of stretched vulcanized rubber as it is affected by different vulcanization accelerators, and variations in extension, temperature and cure. To illustrate a connection between the physical performance of a rubber vulcanizate and its degree of crystallization, measurements were made on the relation of crystallinity to creep when the rubber was stretched to different initial elongations under constant loads. The creep as a function of the elongation has a maximum value at the same intermediate elongation for which crystallization becomes appreciable. At higher elongations, the increased crystallinity results in a diminution of the creep. The ultimate strength and extensibility generally associated with stretched vulcanized rubber is the result of the combined effect of the primary valence cross‐linkages formed by vulcanization and the formation of crystallites caused by stretching. Crystallization is an important factor in maintaining the relatively high strengths of vulcanizates having a greater range of extensibilities. In general, the physical properties and performance of vulcanized rubber is related to the amount of crystalline material formed upon stretching, which of course depends upon the structural characteristics of the vulcanizate.