The effect of the thermal history on the properties of welded high density polyethylene is studied. The lamellar microstructure observed in the weld is different from that in the bulk slow cooled material. The weld has lower crystallinity and smaller lamellar size, both of which change with annealing, The differences in the microstructure between the weld and the surrounding material produce differences in the plastic properties. The low ductility of the welded samples is a direct result of the relatively low yield stress within the weld. Annealing of the weld can improve the tensile properties, but in quenched welds the final properties are still inferior to the bulk material. Displacement controlled, constant velocity microindentation tests are shown to provide a rapid means of evaluating the properties of the weld.