An experimental investigation on the effect of refrigerant liquid injection into the heat pump compressor on the system performance has been carried out. The heat pump in the present study is designed to draw heat from sea water and equipped with a 3-hp reciprocal compressor. The compressor head cover and valve plate has been machined to accomodate two 0.58 mm-I.D. injection nozzles. When the liquid injection mass is increased up to 7.2 % of the refrigerant mass in the cylinder, the discharge gas temperature drops by 39 K. The liquid injection causes the rise of condensing pressure and accordingly compressor power. The COP, however, increases slightly and then decreases, indicating the existence of an optimum injection rate.