•The aspect ratio of manifold microchannel was optimized at fixed mass flux.•The manifold microchannel has the optimal aspect ratio at fixed mass flux.•The optimal aspect ratio was analyzed at various working fluids and solid materials.•A criterion formula of optimal aspect ratio was given. In order to provide a basis for the design and optimization of the aspect ratio of the manifold microchannel, the effect of channel aspect ratio on the heat transfer performance of manifold microchannel heat sinks at fixed mass flow rate is numerically investigated in this present work. The results show that the heat transfer performance of the manifold microchannel heat sink reaches the peaks at one specific channel aspect ratio, which is defined as optimal aspect ratio αo. It is found that the optimal aspect ratio is different at various working fluids and solid materials. The optimal aspect rises with the increase of thermal conductivity of the channel wall and dynamic viscosity of working fluid, or the decrease of specific heat capacity of working fluid and thermal conductivity of working fluid. Further numerical simulations are conducted to quantitatively analyze the influence of each thermophysical parameter on αo. Finally, a criterion formula is given to describe the optimal aspect ratio as a function of Re number, Pr number, and the ratio of thermal conductivity between working fluid and channel wall.