This paper evaluates the validity of a new test methodology for measuring shear fracture toughness (mode II)of high density polyethylene (HDPE). The methodology adopts Iosipescu test for the shear loading, and determines the toughness based on the essential work of fracture (EWF)concept. The results show that even under the Iosipescu loading, tensile deformation (mode I)is still involved in the fracture process, possibly due to the significant work hardening thatHDPEdevelops during the plastic deformation. The study found that the mode II fracture toughness can be determined through data analysis using double linear regression, i.e., by extrapolating specific work of fracture to zero ligament length and zero ligament thickness. The paper demonstrates that the new test methodology can be used to evaluate mode II fracture toughness of ductile polymers like HDPE in which significantwork- hardeningmaybe involved in the fracture process. The paper also provides quantitative comparison of the fracture toughness for HDPE inmode II with its mode I counterpart.