This paper presents several methods for reducing engine weight primarily through substitution with light-weight materials. The efficiency and performance of the engine were reviewed using a light-weight experimental engine (hereinafter called “weight-reduced engine”) constructed by the authors in order to investigate the possibility of practical use of the proposed weight reduction measures.The weight-reduced engine is based on an in-line 4-cylinder, 2.0 liter, gasoline engine with the base engine weight of 162 kg excluding engine oil and coolant and was reduced by 37 kg by applying alternative light-weight materiaLs and new manufacturing techniques. This corresponds to 23 % weight reduction.The materials used in the weight-reduced engine are 53 % steel, 33 % aluminum, 7 % plastics and 7 % other light-weight materials.It was found that by application of light-weight materials, the engine performance of the weight-reduced engine could be improved. That is improvements in response and friction loss. In particular, weight reduction of the major moving components, such as connecting rods, not only contributes to reduce friction loss but leads to a weight reduction for other components.The light-weight components have been put to various kind of testings which cover fatigue of components and bench testing with assembled engine by motoring and firing. These results were sufficient to confirm that these components are durable enough to operate in a practical environment.