The subcooled flow boiling heat transfer of R1233zd(E) in uniform- and variable-width parallel microchannels was investigated experimentally. The heat transfer characteristics of two channel structures were analyzed and compared under different subcooling, mass-flux, and heat-flux conditions. The experimental results reveal that the onset of nucleate boiling (ONB) of the variable-width parallel microchannel occurred earlier and that the wall temperature could be maintained at a lower value. In addition, the overall heat transfer characteristics of the variable-width microchannel were better than those of the uniform-width microchannel. When the mass flux was 1263 kg/(m²·s), the wall superheat of the variable-width parallel microchannel was 3.2 ℃ lower than that of the uniform-width parallel microchannel. When the heat flux was 150 kW/m², the coefficient of heat transfer (HTC) of the variable-width parallel microchannel increased by 30%-50% compared with that of the uniform-width parallel microchannel. Moreover, the two-phase pressure drop in the variable-width parallel microchannels was higher than that in the uniform-width parallel microchannels, with a maximum increase of 61%.