Abstract: Ultra-high molecular weight polyethylene (UHMWPE) fiber reinforced composites are considered as state-of-the-art materials for armor solutions, and interlaminar delamination is one of the main failure mechanisms for the composites under impact loadings. For UHMWPE composite laminates, an improved double cantilever beam (DCB) specimen was proposed. The interlaminar fracture toughness (G_IC) and failure characteristics were then studied. Analysis were conducted regarding the influence of the specimen thickness and fiber layups on the G_IC. The failure mechanism of interlaminar fracture and the effect of structural plasticity on the crack propagation process were further discussed. Evaluation was also implemented on the applicability of the existing test standards for the calculation of the interlaminar fracture toughness. Results show that the DCB specimen with small thickness exhibits obvious plastic behavior, and the measured interlaminar fracture toughness is significantly affected by structural plasticity. Increasing the thickness of the specimen can effectively avoid the influence of plasticity. Conclusively, the results presented in this paper provide experimental reference and data support for the study of dynamic interlaminar properties and theoretical models of UHMWPE composites, that have important engineering significance for the design of composite protective structures.