Network structures have been widely adopted in transportation systems and supply chain systems. Delivery within the promised time frame is especially the most critical quality criterion for supply chain networks. However, little attention has been given to the performance evaluation of on-time delivery for multi-commodity networks. This paper presents a time-constrained multi-commodity multistate flow network (TMMN) which is characterized by (1) each arc employs two attributes, capacity and lead time; (2) the arc capacity is multistate; (3) different commodities consume the arc capacity differently and (4) the delivery has to be completed within the promised time frame. A new method is proposed to locate the optimal routing in a TMMN, as well as to estimate the network reliability of multi-commodity supply chains. The proposed method is targeted towards the situation where multi-commodities are conveyed through all disjointed minimal paths (MPs) in a network. This is the first study that develops a method to locate the most reliable routing in a TMMN and to estimate the network reliability as a performance index for on-time delivery of multi-commodity systems.