Miscible blends of poly(vinyl chloride) (PVC)/poly(α-methylstyrene-acrylonitrile) (α-MSAN) were toughened by polybutadiene-g-poly(styrene-co-acrylonitrile)(PB-g-SAN). The PB-g-SAN content was fixed at 20%, and the ratio of PVC to α-MSAN ranged from 100/0 to 0/100 in the matrix. The dependence of mechanical properties and deformation mechanisms on the entanglement density of rubber-modified PVC/α-MSAN blends was investigated. It was found that the entanglement density of PVC/α-MSAN blends increased with increasing of PVC content. When the entanglement density of the matrix was low, crazing was the main deformation mechanism. For the matrix with high entanglement density, crazing, cavitation, and shear yielding took place simultaneously. On further increasing the entanglement density, only cavitation and shear yielding of the matrix were observed. Therefore, a transition from crazing to shear deformation was observed with the increase of entanglement density in the matrix, leading to higher toughness