Vulcanizates of natural rubber (NR) and its synthetic analogue (IR) were quickly stretched to 6 times the original length. The post stretch relaxation of tensile stress and the development of strain-induced crystallization (SIC) were studied by simultaneous measurements of the stress and the diffraction intensities using the synchrotron X-ray source. In the range of 8 s, NR crystallized much faster than IR. Accordingly, the origin of the superior toughness of NR was thought to come from the ability of rapid SIC. Time constants of the post-stretch crystallization were estimated from the X-ray study. Then the crystallization time constants were used to decompose the contribution of SIC from the total magnitude of the post-stretch relaxation. The contribution of SIC was dominant for the total magnitude of the post-stretch relaxation during several seconds.