For the last ten years, sign languages as alternatives to speech have been applied to severely retarded children as well as autistic children and some of them have been found to be effective. This study was to verify the effectiveness of sign languages applied to severely retarded and autistic children and to see whether or not sign languages learned by these children promote their verbalization. Three methods were compared: 1)simultaneous communication, 2)sign languages only, and 3)oral method. The simultaneous communication method was found to be better than the other two methods in terms of acquistion of sign languages, promotion verbalization and generalizarion.