A model of velocity distribution among microchannels with triangle manifolds is proposed. According to the flow behaviors analyzed by Fluent, the manifolds are divided into several approximate rectangular channels, and then an equivalent simplified resistance network model is developed to establish the relationships between the velocities and pressure drops in microchannels and approximate rectangular channels. The velocity distributions are calculated under two situations, respectively, considering and ignoring singular losses. The outcomes of the present study are compared with Fluent's simulated results to analyze the effects of singular losses on the velocity distributions. It indicates that the proposed model is suitable for the calculation of velocity distribution among microchannels with obtuse angled or right triangle manifolds under low Reynolds numbers. The premise of ignoring singular losses is that the frictional pressure drops are three times larger than the singular pressure drops in each flow loop. The manifold optimization results indicate that the velocity distribution among microchannels with right triangle manifolds is much more uniform than that of the corresponding one with obtuse angled manifolds. (C) 2009 American Institute of Chemical Engineers AIChE J, 55: 1969-1982, 2009