There has been an implicit problem that, in the previous treatments, some behaviors of strain-induced crystallization in cross-linked natural rubber (NR) and synthetic cis-1,4-polyisoprene (IR) were inconsistent with the predictions on the basis of the statistical theory of rubber elasticity. This problem was found to have come from disregard for the existence of the fluid-like network-chain component. In this study, the rubber sample is assumed to be composed of the elastically effective as well as the fluid-like components and their physical quantities are separately considered. The corresponding mechanical model can explain the features of strain-induced crystallization successfully. The mechanical model is assessed using the tensile stress-strain behavior of NR and IR samples at very fast deformation rate.