This study deals with component supply planning in assembly systems, i.e. where several types of components are needed to produce one finished product. The actual component lead times have random deviations, so they can be considered as random variables. MRP approach with Periodic Order Quantity policy is considered. The aim is to find the optimal MRP offsetting. The proposed model and algorithms minimize the sum of the setup and average holding costs for the components, while satisfying a desired service level.