In this paper, the trajectory dispersion of the Viper rocket caused by the elastic vibration of the vehicle and rocket launcher is investigated. In order to estimate the trajectory dispersion, a three dimensional numerical simulation of the launch was carried out considering the elastical effect of the vehicle and rocket launcher. The accuracy of the numerical simulation was validated first by the comparison with the vibration data measured on actual launch, and after that the trajectory dispersion was estimated by the simulation. As the result, it is confirmed that the trajectory dispersion caused by th elastic vibration is no matter on actual launch mission.