The effect of partially stabilized zirconia (PSZ) on the mechanical properties of the hydroxyapatite-high density polyethylene composites was studied by investigating the effect of hydroxyapatite and the simultaneous effect of hydroxyapatite and PSZ volume fractions on fracture strength, modulus of elasticity, and absorbed energy in the composite samples. The results showed a decrease in fracture strength, and absorbed energy with an increase in the volume fraction of hydroxyapatite content in the hydroxyapatite-polyethylene samples. Partial replacement of hydroxyapatite with PSZ particles was beneficial in the improvement of both the fracture strength and failure energy values in the composite samples. A transition from ductile to brittle behavior was observed as the volume fraction of ceramic filler particles increased in the samples.