It is important not only to increase a thermal efficiency of a spark ignition engine, but also to decrease a driving loss of auxiliary component of engine such as a cooling system. For example, there is a limitation of cooling performance when a requirement of aerodynamics design of a motorcycle or an automobile is severe.We propose an active application of ejector pump drove by exhaust energy to the cooling system. There were several applications of ejector system to the intake or exhaust system, but there was nothing for the design of it.In this report, one of the fundamental researches conducted in our laboratory, the effects of the dimensions of the ejector on the ejector performance are discussed.The results are followings: (1)The more the engine speed decreases, the more the ejector efficiency (G2/G1: G1 is the mass of charging air in the engine, G2 is the mass of entrained air by the ejector system) increases. The maximum value of ejector efficiency with four cycle engine is of order 4 to 5, and that with two cycle engine is of order 6 to 8 at the optimized ejector design. This quantity of air is about a half of the cooling air required by the test engine. Therefore, this ejector type cooling system is very attractive auxiliary ones. (2)The more the nozzle diameter of exhaust pipe decreases, the more the ejector efficiency increases. There is no change of a mass of intake air (volumetric efficiency) within the region of [fn]/[fe] ≧ 0.3, where [fn] is the section area of the nozzle, and [fe] is the section area of the exhaust pipe. (3)The maximum value of ejector efficiency is obtained at [fm]/[fe]=5 and [lm]/[dm]=7∼15, where [fm],[lm] and [dm] is the section area, the length and the diameter of the mixing chamber, respectively. (4)The location of nozzle is very important. The maximum value of ejector efficiency is obtained at [a/dm] = 1.0∼1.5, where [a] is the distance between the nozzle and mixing chamber.