In this study, poly(lactic acid) was melt blended with high density polyethylene to investigate the rheological and mechanical properties of these blends. The rheological properties were characterized where viscous properties were evaluated in the molten state. In studying the viscous properties, non-Newtonian index (n), apparent viscosity, and the flow activation energy were determined. Mechanical properties were studied in terms of tensile properties, i.e. ultimate tensile strength, Young's modulus, elongation, and total energy at fracture. The results showed that PLA/HDPE blends and their pure components behave pseudoplastically (n<1), blends apparent viscosity decreases with increasing shear rate. It was also found that the curves of apparent viscosity versus blending ratio go through a minimum at blending ratio PLA/HDPE (75/25) vol% and that the PLA/HDPE (16/84) vol% blend has the lowest value of viscous activation energy. The mechanical results showed that the blends ultimate tensile strength and Young's modulus increased at up to 30 vol% PLA content. By contrast, the elongation and total energy at fracture versus blending ratio showed a continuous decrease up to PLA/HDPE (50/50) vol% ratio. These results comply with the behaviour of an incompatible polymer blend system.