High-performance electronics benefit greatly from the use of microchannel heat dissipation devices, which are sophisticated cooling systems that increase heat transfer efficiency by using microchannels. These devices enable for better cooling performance in situations where space is limited due to their small sizes and high heat transfer coefficients. In this work, 3-dimensional numerical simulations are performed to analyse the effects of varying aspect ratio (AR) from 0.5 to 1.5 in microchannels for enhancing the thermal performance of microchannel heat sinks keeping constant hydraulic diameter (Dh), within the range of inlet velocity (Vin) from 0.05 to 0.45 m/s. Additionally, the performance of circular microchannel is also compared with the rectangular ones having same Dh. The highest temperature difference between rectangular microchannel with AR = 0.5 and circular is 34 K at lowest Vin = 0.05 m/s and 11 K at highest Vin = 0.45 m/s. Circular microchannel shows highest average Nusselt number (Nuavg) and thermo-hydraulic performance at all considered Vin. For rectangular models, microchannel with lowest AR = 0.5 and highest AR = 1.5 show highest Nuavg and thermo-hydraulic performance at lower velocities (Vin < 0.25) and higher velocities (Vin > 0.25), respectively. According to this study, low AR at lower Vin and high AR at higher Vin in microchannels are suggested for thermo-hydraulic performance enhancement in microchannel-based heat dissipation devices.