This study presents experimental frictional pressure drop for R410A/oil mixture flow boiling in small horizontal smooth tubes with inside diameters of 4.18mm and 2.0 mm. Experimental conditions cover nominal oil concentrations from 0 to 5%. The test results show that the presence of oil enhances two-phase frictional pressure drop about 0-120% and 0-90% at present tee conditions for 4.18mm I.D. smooth tube and 2.0mm I.D. smooth tube, respectively, and the enhanced effect is more evident at higher vapor qualities where the local oil concentrations are higher. A new correlation to predict the local frictional pressure drop of R410A/oil mixture flow boiling inside conventional size and small smooth tubes is developed based on local properties of refrigerant-oil mixture, and the experimental data of 4.18mm I.D. and 2.0mm I.D. smooth tubes and that of 6.34mm I.D. smooth tube (Hu et al., 2008) are well-correlated with the new correlation.