Under uniaxial tension, high-density polyethylene (HDPE) often fractures in a ductile manner. The fracture involves extensive necking that is known to occur in the plane-stress condition. However, for large-scale HDPE products like polyethylene pipe, crack can grow rapidly in a brittle manner. This type of fracture is known to be in the plane-strain condition, which has much lower toughness than that in the plane-stress condition. In this study, we extended the concept of essential work of fracture (EWF) to the plane-strain condition and evaluated the fracture toughness of extruded HDPE plate. The results show that the specific work of fracture in the plane-strain condition is also a linear function of the specimen ligament length. Therefore, the corresponding EWF value can be determined in a similar manner, that is, through linear extrapolation to zero ligament length. For HDPE, the study found that the plane-strain toughness is about one order of magnitude smaller than the plane-stress counterpart. Interestingly, the plane-strain EWF value is very close to the estimated toughness of pressurized polyethylene pipe, which was reported in the literature, predicted using numerical simulation based on data from the standard small-scale steady-state (S4) test.