This paper discusses the effect of lubricant oil on nucleate boiling performance for a smooth tube. The lubricant oil used in the investigation includes 3GS and 5GS, and the working refrigerant is R-22. Experiments were conducted at saturation temperatures of -5°C, 4.4°C, and 20°C. For refrigerant-oil mixtures, the heat transfer coefficients, h, decrease with increase of oil concentrations at Ts = 20°C. Conversely, for Ts = -5°C, the heat transfer coefficients for refrigerant-oil mixtures may be larger than those of pure refrigerants at low oil concentration. Possible explanations for this phenomenon include (1) the effect of physical properties, especially that of surface tension, and (2) foaming characteristics.