Characteristics of strain-induced crystallization (SIC) are described on sulfur cross-linked and peroxide cross-linked natural rubber (NR) and synthetic isoprene rubber (IR). Simultaneous tensile and wide-angle X-ray diffraction measurements using synchrotron radiation systems were carried out in order to elucidate SIC. The stretched molecular chains acted as initiating species of the crystallization, while surrounding chains contributed to the crystal growth. Stretching ratio at the onset of SIC (α_c) decreased with an increase of network-chain density (ν) for peroxide cross-linked NR (P-NR), while it remained constant for sulfur cross-linked NR (S-NR). But, dependence of relative crystallization rates on ν was similar for both P-NR and S-NR. Calculated entropy differences between the undeformed and the deformed states at α_c were equal for P-NR regardless of ν, whereas it became smaller with the increase of ν for S-NR. The SIC behavior of P-NR was in agreement with the prediction on homogeneous or uniform networks by Flory. Network in sulfur cross-linked IR (S-IR) was composed of domains of high ν value embedded in the rubbery network matrix, which gave a characteristic feature in its SIC. Mechanical propeties of S-NR and P-NR are also discussed from the viewpoint of their SIC behaviors on the basis of the network structures.