This study investigates the exergetic efficiencies of plain microchannel heat exchangers under single-phase and two-phase operating conditions using de-ionized water as the working fluid. The friction factor and heat transfer coefficient data of microchannels are obtained using experimental studies from the literature and analytical/empirical correlations. The exergetic efficiencies of four plain microchannels are compared in order to reveal the effect of different geometrical parameters in terms of exergetic efficiency. The results showed that the microchannel aspect ratio (width/height) is an insignificant parameter on the exergetic efficiency for single-phase flow conditions. However, the exergetic efficiency increased with the decreasing of the microchannel aspect ratio in two-phase tests. On the other hand, the microchannel heat exchanger having shorter heated length exhibited better exergetic efficiency both in single-phase and two-phase regimes compared to the microchannel heat exchanger with longer heated length.