The flow boiling heat transfer of refrigerant 12 in an electrically heated horizontal tube is investigated, The experimental results indicate that the heat entrance effect must be taken into account for flow bolling heat transfer in the tube. The early expertmental data which, neglected the entrance effect are larger than the real values. The investigations also illustrate the effect offlow patierns on non-uniform heat transfer around the perimeter of the tube. The perimeter averaged heat transfer coefficients depend on heat flux, mass flow rate, quality and saturated pressure. It is proved that heat transfer of flow boiling consists of the fully developed nucleate boiling heat transfer and the convcctive evaporation heat transfer. The experimental data are in good greement with some existing correlations in the foreign literature.