Aviation line replaceable unit ( LRU) is an essential support unit of aircraft maintenance. Firstly, the LRU could be divided to three levels, line replaceable unit ( LRU) , line maintenance part ( LMP) , shop replaceable unit ( SRU). Then, based on the research of the factors of the aviation material inventory costs, the aviation material inventory costs include ordering costs, holding costs, shortage costs and repairing costs. The storage costs include in the holding costs, which have the positive related to the price, quantity and type of the aviation material. Thus, the LRU division factor have been proposed, the optimal LRU hierarchical model was established, based on ensuring the guarantee rate of aviation material and minimized the inventory costs. Taking the actual operation data of an airline as an example, the LRU division level of one hydraulic system pressure module assembly was determined. Finally, because the classification and quantity of aviation material increase, the airlines management costs will be increased by decreasing the LRU division level from the point view of aviation inventory costs.