A product life cycle is the life span of a product which the period begins with initial product specification and ends with the withdrawal from the market of both the product and its support. A product life cycle can be divided into several stages characterized by the revenue generated by the product. This paper deals with an inventory model for Product Life Cycle with defective items for single manufacturing system which consists of introduction, growth, maturity and decline stages. The defective rate is considered as a variable of known proportions. The main objective is to minimize the total net inventory cost and optimal quantity in product life cycle. The relevant model is built and solved. Necessary and sufficient conditions for a unique and global optimal solution are derived. A numerical example is provided and numerically verified. The validation of result in this model was coded in Microsoft Visual Basic 6.0.