To investigate the influence of hydraulic diameter on fluid flow and heat transfer mechanisms in rectangular microchannel heat sinks, ultrafast laser technology was employed to fabricate six microchannel structures with varying geometric dimensions. Deionized water and ethylene glycol solution served as the experimental working fluids. The flow and heat transfer characteristics were examined in microchannels with hydraulic diameters ranging from 0.5 mm to 1.2 mm, under operating conditions of volumetric flow rates between 40 and 60 L/h and heat fluxes from 100 to 800 kW/m². The findings indicate that the hydraulic diameter plays a crucial role in governing fluid flow and heat transfer behaviors within microchannels. Under the same experimental conditions, using deionized water as a reference, the Nusselt number exhibits a steady increase with hydraulic diameter, reaching a maximum enhancement of 56.32%. Conversely, the pressure drop is observed to decrease with increasing hydraulic diameter, with a maximum reduction of 83.53%. Moreover, both the Nusselt number and the pressure drop are found to rise alongside the Reynolds number. The proposed heat transfer prediction correlation demonstrates robust accuracy, with average absolute errors (MAE) between the predicted and experimental values for microchannels with hydraulic diameters of 0.5 mm/M1, 0.8 mm/M2, 1.0 mm/M3, 1.0 mm/M4, 1.2 mm/M5, and 1.2 mm/M6 being 13.93%, 13.91%, 13.46%, 13.83%, 11.34%, and 14.56%, respectively (M1, M2, M3, M4, M5, M6 are microchannel heat sinks). The predicted values consistently fall within the ±15% error band.