We investigated properties of the relativistic terahertz radiation from an underdense plasma perturbation that is induced by high-power fs laser. In particular, this study is focused on the effect of relativistic Lorentz factor on the radiation frequency, for which extensive PIC (particle-in-cell) simulations were performed. In addition, the possibility of tuneable and controllable spectrum generation by interactions of an intense laser beam and a plasma was studied. In this poster, the theoretical and simulation results are presented.