The effect of a rubber mould on densification and deformation of aluminum alloy powder was investigated during warm isostatic pressing. The hyperelastic constitutive equations based on various strain energy potentials were employed to analyze deformation of rubber. The parameters in the strain energy potentials were obtained from experimental data for uniaxial and volumetric compression of Viton rubber at two warm temperatures. For elastoplastic response, the yield function of Shima and Oyane was implemented into a finite element program (ABAQUS) to predict compaction responses of metal powder during warm isostatic pressing. Finite element results were compared with experimental data for densification and deformation of aluminum alloy powder with/without a rubber mould under warm die pressing.