The purpose of the experiment is to study the effect of oil in refrigerant on mass flux in a capillary tube. A capillary tube 1.50 m long with inside diameter of 0.6 mm is used for testing in a HFC-134a refrigeration system. The tests are conducted over a range of inlet subcooling from 3.20 °C to 19.26 °C. The experiment shows that there is a difference between mass fluxes with and without oil. A relation is given to estimate the mass flux decrease with oil in capillary tube. The results indicate that influence of oil on the performance of capillary tube should be taken into consideration in design of refrigeration systems.