Experiments on flow pattern and flow boiling heat transfer of CO₂ and CO₂-oil mixtures in horizontal small diameter smooth and micro-fin tubes were carried out. The test smooth tube is a stainless steel tube, 2.0mm in inner diameter and 1m in length. And the test micro-fin tube is an inner grooved copper tube, 1.95mm in mean inner diameter and 1 m in length. Experiments were conducted at mass velocities from 380 to 1500 kg/(m²s), a saturation temperature of 10°C, heat fluxes from 10 to 43.3 kW/m². The flow patterns in both the tubes were observed and the results were compared with the flow pattern map proposed by Cheng et al. for smooth tubes. And the measured local heat transfer coefficients for the smooth and micro-fin tubes were compared with those calculated from the prediction correlations proposed by Cheng et al. for smooth tubes. As the results, the flow patterns in both the tubes correspond well with the flow pattern map. On flow boiling heat transfer of pure CO₂, the prediction model agrees well with the experimental data of the smooth tube, and the measured local heat transfer coefficients of the micro-fin tube are about 31% higher than those of the smooth tube. For CO₂-oil mixtures, the measured local heat transfer coefficients of both the smooth and micro-fin tubes are much lower than those of pure CO₂. The dryout inception qualities of the micro-fin tube increase with the increase in mass velocities, while those of the smooth tube decrease with the increase in mass velocities. The dryout inception qualities of the micro-fin tube are much higher than those of the smooth tube.