For reduction of the weight of propellants for Reaction Control Systems (RCS), and also increase of efficiency of RCS, we have conducted two methods of the injection which make jet interaction more effective. The first method is a periodic injection method and the other is the injection method with multiple ports. Those two injection methods are simulated by using the two or three dimensional Reynolds Averaged Navier-Stokes equations coupled with Wilcox's k-ω turbulence model. In a periodic injection, the high pressure region is kept for 50μsec just after the shut-down of injection. In the injection method with multiple ports, the results show that amplification factor increase by 20 % compared with single injection with same mass flow rate.