The main aim of this thesis is to develop a framework that allows an equipment manufacturer to forecast the demand of its aftermarket user-replaceable parts and prepare the challenges it may face from third party manufacturers. Some business practices ensuring the performance against third party part manufacturers are suggested in this thesis as a reference for a manufacturer to retain its customer and optimize its performance in aftermarket. The first part of this thesis focuses on the framework of a forecasting model for an equipment manufacturer which needs to provide aftermarket consumers to end users through the service or sales channels of the manufacturer. The forecasting model is carried out using the data from annual sales quantity of one model of equipments(known as install base) during the product life cycle. Each year during the product life cycle would have a different Rate of Replacement. The forecasting model aggregates the data from the install base and the rate of replacement to come up with an annual demand of the aftermarket replacement parts needed for the equipment. The second part of this thesis focuses on some practical business practices for the manufacture to ensure its performance in aftermarket. The designs of the products, logistics arrangement and selection of channels are also curial to its performance.