Flow boiling heat transfer coefficient of R-22 in small-diameter round tubes has been experimentally studied to investigate the effect of tube material. The experimental apparatus consists mainly of a refrigerant pump, a condenser and a receiver, two test sections of 300 mm-long round tubes and a pre-heater for control of refrigerant quality at the inlet of test section. The ranges of the major experimental parameters in this study used are 600 kg/m2s of refrigerant mass flux, 5～30 kW/m2 of the wall heat flux and 0.0～0.9 of the inlet vapor quality. The experimental result showed that the boiling heat transfer coefficient in small tubes are in the range of 1～5 kW/m2K and it varies only by heat flux, independent of mass flux and vapor quality. The flow boiling heat transfer coefficients in three different tubes of copper, aluminum and brass, have been obtained and the results show that the effect of tube material is small.