This paper deals with the simulation and optimisation of disassembly-reconditioningassembly system (DRAS) taken into account random machines failures and repairs. The proposed system is composed of one disassembly machine, two parallel structures, one assembly machine and stocks to store the used products, components and finished product. In this study a complete disassembly and assembly process in a supply chain is considered, and which allows to acquire used products, disassemble the used products, storing reconditioning components, assemble the components and selling the finished product to reply customers demand. The objective is to determine the optimal acquisition quantity of used products that maximises the total profit. A mathematical model is formulated on the basis of discrete flow model that allows to express the behaviour of the system. The proposed model is then simulated and the optimal acquisition quantity is determined. Numerical results are presented to discuss the optimal acquisition quantity and the lost profit risk.