The two-phase flow patterns of HFC-134a with lubricant oil mixtures inside a smooth horizontal tube were experimentally elucidated. Tests were performed in an inside diameter of 7.8 mm having a lubricating oil concentration of 5%. Tests were made of mass fluxes ranging between 150 and 590 kg/m(2) s. The most obvious difference from oil-free cases reported is the presence of froth flow pattern. Apparently, this flow pattern is related to the increase of surface tension and viscosity. With the presence of lubricant oil, the onset of transition from stratified flow region to annular flow regime shifted to a lower value of superficial gas velocity. In addition, the tearing phenomenon of the refrigerant-oil mixtures may be related to its relevant properties such as wettabilitly and surface tension.