The cross-link density of DCP-cured natural rubber uniaxially deformed to various extension ratios at room temperature was measured. The cross-link density decreased characteristically with increasing extension ratio. A new model for the structure of the network is proposed in order to explain this decrease in cross-link density. The model can give a new interpretation of the stress-strain behavior of rubber vulcanizates. The birefringence of stretched rubber vulcanizates was determined using a polarizing microscope. The observed birefringence, δT, can generally be factored into two parts: One is the birefringence of the amorphous phase, δTa; the other is the birefringence due to the strain-induced crystalline phase, δTc. In the present paper, it has become feasible to separate δT into δTaand δTc.