This paper deals with the experimental study on flow boiling heat transfer of carbon dioxide in a micro-fin tube. The geometrical parameters of micro-fin tube used in this study are 6.07 mm in outer diameter, 5.24 mm in average inner diameter, 0.256 mm in fin height, 20.4 in helix angle, 52 in number of grooves and 2.35 in area expansion ratio. Flow patterns and heat transfer coefficients were measured at 3-5 MPa in pressure, 300-540 kg/(m<Sup>2</Sup>s) in mass velocity and -5 to 15 °C in CO<Sub>2</Sub> temperature. Flow patterns of wavy flow, slug flow and annular flow were observed. The measured heat transfer coefficients of micro-fin tube were 10-40 kW/(m<Sup>2</Sup>K). Heat transfer coefficients were strongly influenced by pressure.