We investigate experimentally various aspects of the crack propagation phenomena in elastomers by means of “dc/dt” method. Propagation speed (v), shape of crack tip, and local strain distribution near the crack tip are examined as a function of imposed tearing energy (Γ). In particular, we focus on a discontinuous transition of v at a characteristic value of Γ (Γ_c) between slow and fast modes. We correlate the Γ dependence of v and crack-tip shape with macroscopic nonlinear viscoelasticity of the elastomers. The change in the crack-tip shape accompanying the velocity transition is explained by weakly nonlinear fracture mechanics with the ratio of the first- and second order elastic moduli. The power law exponent for the Γ dependence of v (Γ ~ v^α) in fast mode has a close correlation with the exponent for the viscoelastic spectrum in glass-rubber transition regime (G(t) ~t^-K). We find Γ_c is governed by the product of the fracture toughness and the ratio of the first- and second order elastic moduli for the elastomers. We also show that the local strain distribution near the crack tip in dynamic crack revealed by DIC technique involves the strong effect of nonlinear elasticity.