Experiments were carried out on the flow pattern, heat transfer, and pressure drop of flow boiling of pure CO 2 and CO 2 -oil mixtures in horizontal smooth and micro-fin tubes. The smooth tube is a stainless steel tube with an inner diameter of 3.76 mm. The micro-fin tube is a copper tube with a mean inner diameter of 3.75 mm. The experiments were carried out at mass velocities from 100 to 500 kg/(m 2 β
s), saturation temperature of 10 Β°C, and the circulation ratio of lubricating oil (PAG) was from 0 to 1.0 mass%. Flow pattern observations mainly showed slug and wavy flow for the smooth tube, but annular flow for the micro-fin tube. Compared with the flow patterns in the case of pure CO 2 , an increase in frequency of slug occurrence in the slug flow region, and a decrease in the quantity of liquid at the top of the tube in the annular flow region were observed in the case of CO 2 -oil mixtures. With pure CO 2 , the flow boiling heat transfer was dominated by nucleate boiling in the low vapor quality region, and the heat transfer coefficients for the micro-fin tube were higher than those of the smooth tube. With CO 2 -oil mixtures, the flow boiling heat transfer was dominated by convective evaporation, especially in the high vapor quality region. In addition, the heat transfer coefficient decreased significantly when the oil circulation ratio was larger than 0.1 mass%. For the pressure drop characteristics, in the case of pure CO 2 , the homogeneous flow model agreed with the experimental results within Β±30% for the smooth tube. The pressure drops of the micro-fin tube were 0-70% higher than those predicted with the homogeneous flow model, and the pressure drops increased for the high oil circulation ratio and high vapor quality conditions. The increases in the pressure drops were considered to be due to the increase in the thickness of the oil film and the decrease in the effective flow cross-sectional area.