Abstract: According to the influence caused by the disabled equipment that a spare part was installed,demand for spare parts could sometimes be classifieded into critical and non-critical demands.This paper dealt with a stochastic inventory model with Poisson demand,a deterministic lead time and a lot-for-lot stocking policy.We put forth a new policy where some of the stock was reserved for critical demand and had derived an algorithm to approximate the service levels for both classes of demand.The case study of a power plant showed the advantages of using the new policy.

Key words: spare parts, demand classes, (S-1,S)policy, critical inventory level