For efficient scale up of microchannel devices, uniform flow distribution to parallel microchannels is very important. In the present work, a dimensionless geometric parameter indicating the optimal manifold configuration is derived through a numerical parametric study and validated by micro particle image velocimetry. It is clearly shown that the flow uniformity is maximized when the dimensionless parameter is about 0.8. This study presents a simple and systematic design methodology for a uniform flow distribution to multiple microchannels operating in parallel.