The flow boiling heat transfer of R123/R134a non-azeotropic mixture has been experimentally studied in a single microchannel of 190 μm ID for various experimental conditions: mixture composition (R123 ratio: 0.250, 0.502, 0.746), mass velocity (G = 314–470 kg/m2 s), heat flux (q″ = 10–20 kW/m2) and vapor quality (x = 0.2–0.9). The wall temperature is measured by a thermometer attached to the wall of the microchannel. Fluid temperature is measured at the inlet and outlet of the microchannel. The heat transfer coefficient is calculated based on the temperature, pressure, and mass flow measurement. The results indicate that the heat transfer characteristics of the mixed refrigerant are a function of mixture composition. The heat transfer for R123 mole fractions of 0.502 and 0.746 show good agreement with those resulting from the heat transfer mechanism of thin film liquid evaporation as pure R123. On the other hand, the heat transfer characteristics for R123 mole fraction of 0.250 indicates a heat transfer mechanism of nucleate boiling followed by two-phase forced convection of pure R134a. The criterion for the heat transfer mechanism concerning the flow boiling of mixed refrigerants in microchannels is determined by the presence of nucleate boiling at low vapor quality, which is identical to the criterion for pure refrigerants. The criterion of the heat transfer mechanism for mixed refrigerants is verified by the comparison with experimental results. Finally, the flow boiling heat transfer correlation for a mixed refrigerant in microchannels is developed from that for pure refrigerant using a correction reflecting the mass transfer effect in the mixed refrigerant. The new heat transfer correlation makes good predictions for experimental data. The new correlations can be applied to refrigerants which have similar thermophysical properties with R134a or R123.