Regarding the penetrating distance of fuel spray in a diesel engine, the old theory dealing with the motion of a fuel droplet in still air is recognized not to coincide with the actual phenomenon in a diesel engine because of the extremely small size of atomized fuel droplets and the very high density of gas in cylinder. In this paper, the characteristics of spray penetration are discussed from the viewpoint of momentum theory based on the idea that the air induced into a fuel jet stream makes a kind of mixed gas together with fuel droplets. According to the results of experiment, the authors confirmed that the theory was satisfactory, that there existed the simple relations among several dimensionless numbers which indicate the effect of various factors on the spray penetration, that there is a close relationship between the spray cone angle and penetration, and others.