This paper studies the use of safety stock in single-item inventory systems. At first , we studied the rela2 tionship of two kinds of service level , and then based on this relationship , by numeric arithmetic , we constructed a search procedure to find the optimal order quantity. The cost implications of using these criteria are thoroughly ex2 plored , the major criteria in determining and evaluating safety stock are identified. The issues of discrepancy be2 tween the safety stock use in theory and that in practice are addressed. At last we explored an efficient heuristic de2 cision of selecting the safety factor for managers to cope with uncertainties under different user’s environment