Abstract: A method for providing inventory optimization for levels of products in a complex supply chain network for multiple internal supplier or manufacturer locations and external distributor or retailer locations. The invention constructs a representative supply chain network model to indicate the flow of products between internal and external locations, it determines inventory levels and fill rates to meet the service level requirements, calculates a total inventory cost for all products in the network, and optimizes the fill rates based on estimated gradient information of the total inventory cost.