For air-cooled heat exchangers of air-conditioners, it has become necessary to improve heat transfer coefficient in tubes in order to balance the other improvements, i.e. higher air-side heat transfer area. In the mean time the increase of pressure drop has been inherent in the increase of heat transfer coefficient in tubes. This paper shows that a new heat transfer tube can increase boiling heat transfer coefficient in tube to about twice that of a smooth tube, without an increase in pressure drop. For a horizontal tube about 11.2 mm I.D. with small internal spiral grooves using Refrigerant R-22, the optimum size of internal grooves ranges as follows, Depth of the groove 0.2mm Pitch of the groove 0.51.0mm Inclination of the groove 7° or 90°