In order to scrutinize the coolant mass distribution and its effect to the heat transfer in oblique fin microchannel array, extensive numerical studies are performed on planar oblique fin configuration. Full-domain simulations using common-flow down (CFD) approach are employed to provide better insights into the flow distribution, flow stability, and heat transfer performance at a global level. The flow field and temperature profile analysis shows that nonuniform coolant distribution and coolant migration occur in the oblique fin microchannel, and the heat transfer performance for both edges of the heat sink is affected due to changing secondary flow rate. However, the flow migration does not affect the local coolant velocity and temperature profiles significantly in the middle region (0.2 < Z′ < 0.8). Meanwhile, it is also found that Reynolds number affects the coolant migration, the stability of the fluid flow, and heat transfer performance significantly. Higher Reynolds number increases the percentage of secondary flow rate and, hence, enhances the heat transfer for fin surfaces in secondary channels.