The scattering of the radar signal by the coated target of a rough surface in the terahertz wave is studied. Considering the roughness of the surface, reflection coefficient is modified for the rough surface. The modified equivalent current can be obtained by using the reflection coefficient. Then using the modified equivalent current and the physical optics solution, the radar cross section of the coated object with the rough surface is obtained. And combining the graphical electromagnetic computing method, simulations to validate the formulation are performed. Numerical results for several coated bodies such as cylinder and cone-sphere geometry are given and discussed from different angles of incidence, different frequencies, different materials, different roughness values, and different coating thickness values.