The present study investigates the microchannel heat sink for pure electroosmotic, pressure-driven, and mixed (electroosmotic and pressure-driven) flows. A three-dimensional numerical analysis is performed for electroosmotic and mixed flows. Electroosmotic flow (EOF) induced in an ionic solution in the presence of surface charge and electric field is investigated with hydrodynamic pressure-driven flow (PDF) to enhance heat removal through the microchannel heat sink. In a pressure-driven microchannel heat sink, the application of an external electric field increases the flow rate that consequently reduces the thermal resistance. The effects of ionic concentration represented by the zeta potential and Debye thickness are studied with the various steps of externally applied electric potential. A higher value of zeta potential leads to higher flow rate and lower thermal resistance, which consequently reduce the temperature of the microprocessor chip and load of the micropump used to supply coolant to the microchannels.