Cavitation instability in rubber is investigated by examining spherical void expansion in rubber particles under dead-load traction conditions. Spherical symmetry is assumed to simplify the governing equations in order to gain qualitative understanding of cavitation phenomenon. A simple strain failure criterion for rubber is proposed to demonstrate the effect of rubber failure on cavitation phenomenon. When the strain failure criterion is considered, the results show that, as in neo-Hookean materials, critical cavitation stresses exist for Mooney-Rivlin materials and for nonlinearly elastic materials characterized by a third-order strain energy function.