This paper studies boiling heat transfer characteristics of the R744 coolant. The heat transfer characteristics comprise the boiling heat transfer coefficient and the flowing pattern inside the vapor tube. The R744's boiling heat transfer coefficients were measured for different filling ratios, and the results show that within the 107-138% filling ratio range, the heat transfer coefficient is almost the same for different filling ratios, but increases with increasing the heat flux. Next, the heat transfer coefficient at the high pressure region is measured. For the same filling ratio and heat transfer capacity, increasing the pressure increased the heat transfer coefficient. The boiling heat transfer coefficients and the flowing patterns for different coolants is compared, including R134a, R410A, and R744, and the boiling heat transfer coefficient of R744 was much higher than those of the other two coolants. In addition, for R744, the diameter of bubbles at the exit point of the evaporator was also smaller than those for R410A and R134a. Finally, we compared our experimental results with existing empirical results, and developed our own empirical formula based on our experimental results. (C) 2017 Elsevier Ltd. All rights reserved.