Abstract: An ultrasonic vibration field can be used to enhance the heat transfer efficiency in microchannels. This study aims to investigate the flow-boiling heat transfer characteristics of nanofluids in the microchannels with or without ultrasonic wave. A microchannel test was designed for the section that can be placed in an ultrasonic transducer. An ultrasonic vibration method was selected to prepare the uniform and stable TiO2/R141b nano-refrigerant with the mass fraction of 0.1%, 0.2% and 0.3%. The flow-boiling parameters of nanofluid were measured in the microchannels under ultrasonic wave. A flow boiling experiment was performed on a rectangular microchannel with a cross-sectional width of 2 mm, where the design system pressure of 152 kPa, the effective heat flux density ranged from 10.8 to 22.7 kW/m2, the ultrasonic power of 50 W, ultrasonic frequency of 23 kHz, mass flow rate of 121.1 kg/(m2·s), and inlet temperature of 35 ℃. Different enhancement effects of heat transfer can be achieved under the nanofluids with different mass fractions. The reason is that the nanoparticles can be used to enhance heat transfer, while increase the thermal resistance avoided by heat transfer, and thereby the increase of thermal resistance can reduce the heat transfer efficiency. The results show that the heat transfer coefficient reached the highest, when the mass fraction of nanoparticles was 0.2%, where the heat transfer enhancement effect can be the best. The nanofluid with a mass fraction of 0.2% under the action of ultrasound indicated the optimal enhancement effect of heat transfer, compared with the case of no ultrasound, where the average saturation boiling heat transfer coefficient of R141b increased by 89.9%. The heat flux posed a great influence on the enhanced heat transfer effect of ultrasonic. There was a significant difference in the enhancement effect under different heat fluxes. The average saturated boiling heat transfer coefficient of nano-refrigerant under the action of sound field increased first and then decreased, with the increase of effective heat flux density. The sound field of vapor-liquid interface in the microchannel was also simulated by COMSOL software. The simulation results show that the propagation of ultrasonic waves in the bubble was weak. When the effective heat flux density below 15.2 kW/m2, the ultrasonic wave can enhance the heat transfer via the increase in the breakaway frequency of the bubble, whereas, the average saturated boiling heat transfer coefficient increased, as the effective heat flux density increased. After the effective heat flux density was 15.2 kW/m2, the strengthening effect of ultrasound began to weaken, due to the increase of bubbles in the microchannel. When the effective heat flow density reached 19.8 kW/m2, the change of flow pattern can lead to an uniform heat transfer, due mainly to the elongation of bubble flow in the microchannel. In the nano-refrigerant with a mass fraction of 0.2%, the enhanced heat transfer effect increased successively for the imported ultrasonic wave, and the ultrasonic wave of the inlet and outlet. When the ultrasonic wave was applied to the inlet, the average saturated boiling heat transfer coefficient increased by 26%, whereas, increased by 46% under the action of ultrasonic import and export. The findings can provide new ideas to improve the heat transfer performance of microchannels when applying ultrasonic waves.