Abstract: Influence of oil on flow condensation frictional pressure drop of R410A inside 5 mm and 3 mm O.D. horizontal smooth tubes was investigated experimentally. The research results show that the presence of oil always decreases the flow condensation frictional pressure drop of R410A inside smooth tubes, and the decrement can reach up to 19% at the present test conditions; the decrement increases with the increase of nominal oil concentration, and decreases with the increase of mass flux and vapor quality. A new correlation of frictional pressure drop for R410Aoil mixture flow condensation inside small diameter smooth tubes was developed based on the mixture properties, which agrees with all the experimental data within the deviation of -20% ~ +30%.