The service life of rubber products depends on the resistance of fatigue crack growth. It was confirmed that the R-ratio is an important influencing factor on the fatigue life of natural rubber and this could related to strain induced crystallization. With increasing R value (the fraction of minimum load and maximum load), the life time of natural rubber can be significantly increased. In this study the characteristic crack growth path related with R-ratio (R = ε minmax) has been investigated. Pure shear samples for filled and unfilled natural rubber are used. It is obvious that varying R-ratio changes the crystallization zone in front of the crack tip.