As a means of effectively incorporating the concept of “life cycle” for reducing the environmental impact of the automobile, we carried out a life cycle inventory study on a part-by-part basis.The targets of our study are the fuel tanks that are made of different materials and manufacturing processes. One is made of steel, and the other is made of plastic, both perform identical functions. Our evaluation study encompasses the period from the manufacturing of the main materials until the disposal of the tanks.The evaluation items consist of the amount of energy consumed and the emissions (of CO2, NOx, SOx, and PM) that are released into the atmosphere. The results show that the plastic tank poses a greater burden in terms of the amount of energy consumed and the CO2 and NOx emitted. This study confirmed that the areas to be improved in the life cycle of the tank, which could not be adequately assessed through the life cycle inventory analysis of the entire automobile, can be effectively extracted through the life cycle assessment of the tank itself.Lastly, we considered the issues that we should address in order to render the life cycle assessment (LCA) into practical application.