High-pressure drops characteristic of microchannel heat sinks (MCHS) is an issue that needs to be addressed to reduce the size of heat-removing devices in compact electronic devices. Supercritical carbon dioxide (s-CO₂) is a suitable candidate being proposed as an alternative coolant to enhance the cooling of the microchannel heat sink (MCHS), with high heat flux, due to its favorable thermophysical properties near its critical point. In this study, numerical simulations are conducted to evaluate the thermal and hydraulic performance of a channel for a designed heat sink with s-CO₂ (at constant (Formula presented.)) and compare it with conventional liquid coolant (water). The effect of coolants mass flow rate ((Formula presented.)), channel aspect ratio (AR), and inlet temperatures on the thermal and hydraulic performance of one channel is studied by varying (Formula presented.) from 0.004 to 0.03 kg/s and AR from 0.33 to 10. The results show that, for the same aspect ratio, same geometry, and constant heat flux, s-CO₂ offers a higher overall heat transfer coefficient (32%) with a lower friction factor (pumping power) compared with the water at the same inlet temperature (T = 32°C). The results of pumping power comparison between two coolants reveal that for CO₂ in supercritical conditions ((Formula presented.), T = 32°C), the consumed power varies by change of the aspect ratio, which is 1.85 times lower than water for AR = 0.33 and is 3.6 times higher for AR = 10. However, in the subcooled state, the reverse effect of the aspect ratio is seen.