Abstract: Based on elastic-plastic fracture mechanics theory, combined with mixed-mode I/II rupture test of UPVC (unplasticised polyvinyl chloride) under multiple loading angles, the composite fracture toughness of UPVC is studied. The critical CTOA (crack-tip-opening angle) (Ψ_C1) at stable composite crack extension is determined by using the digital correlation image technique, and Ψ_C1 is used as the fracture toughness parameter to describe the fracture toughness of UPVC. It is shown that: (1) unlike the common metallic materials, the load-carrying capacity of UPVC under mode II loading conditions is the largest, while under mode I loading conditions it is the smallest; (2) the CTOA tends to be stable when the crack extension is steady, so it can be used as the fracture toughness parameter of UPVC.