The distance between cylinder axes (D.B.C.A.) Is a fundamental factor for delimiting the engine configuration of all wheeled and caterpillar vehicles, shorter the length, more compact the engine overall size. It is also the essential dimension of manufacturing equipments for machining the main components of engine, and will be hardly ever changed once fixed. But it was merely determined by layout drawing in the past. This paper details three main factors for determination of D.B.C.A.. A series of semi-empirical formula were established using for the engine layout design after other factors affecting the design were carefully considered.