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This paper presents an experimental and theoretical investigation on two-phase flow boiling heat transfer with carbon dioxide (CO 2 or R744) as the refrigerant in horizontal small diameter tube at low temperature. The experimental data were obtained in the following condition, the heat flux is 7.5–30 kW/m 2 ; the mass flow rate is 300–600 kg/m 2 s; the saturation temperature is 233–273 K; the test section is made of stainless steel tubes whose inner diameters are 0.6 and 1.5 mm. The experimental results show that heat transfer coefficient increases with the increase of mass flow rate, but decreases with the increase of tubes’ inner diameter and saturation temperature, the boiling heat transfer of CO 2 has a greater effect on nucleate boiling and highly depends on heat flux. The comparative study indicates that: during the existed flow boiling heat transfer model, the model which has the best prediction accuracy presented with 79.8% of predicted points within ±30% and 21.8% mean deviation before the dryout phenomenon happened, while it was only 18.4% and 59.9% after the dryout phenomenon happened. [ABSTRACT FROM AUTHOR]