The Korean telecommunications market has been expanding swiftly, these days, to be saturated. In this environment, the upcoming mobile telecommunication market, where 4G service was introduced this year, is becoming more substitutive and competitive. Thus, the demand forecasting of 4G service is very difficult, while it is critical to market success. This paper adopts a multi-generation diffusion model to capture the diffusion and substitution patterns for two successive generation of technological services, i.e., 3G and 4G mobile telecommunications services. The three parameters, i.e., the coefficient of innovation, the coefficient of imitation, and the coefficient of market potential, used in the multi-generation diffusion model based on Norton and Bass[11] are obtained by inference from similar substitutive relations between older and newer telecommunication services to 3G and 4G services. The simulation results show that the Bass type multi-generation model can be successfully applied to the demand forecasting of newly introduced 4G mobile telecommunication service.