A design methodology is proposed to maintain gas and liquid flow nonuniformities below an acceptable limit in a parallel micro/millichannels reactor by determining the maximum allowed temperature deviation in each part of the reactor. The effect of temperature deviation on flow distribution was quantified using a hydraulic resistive network model. The effect of flow rate on temperature deviation was demonstrated using a one-dimensional energy balance model. Experiments were conducted using the barrier-based micro/millichannels reactor (BMMR). Flow distribution in the BMMR is based on placing hydraulic resistances (barrier channels) in the gas and liquid manifolds to regulate the flows. Temperature deviation in the barrier channels affects flow nonuniformity by 10 times more than in the reaction channels. Above a certain critical liquid residence time, flow rate has no significant effect on the temperature deviation which depends on the liquid used, reactor material of construction, and its geometrical dimensions.