In the calculation of the rough surface, one of the methods to establish the model is to view the rough surface as a plate with hemispherical bosses. Thus the calculation of a hemispherical boss becomes a part of the simulation of the rough surface. With the development of radar technology, the requirement of the radar cross section (RCS) becomes higher, which makes the measurement and calculation of the terahertz RCS become more important. However, due to the different energy distribution between the Gaussian beam and plane wave, the simulation results would exist some error when assuming that the incident beam is a plane wave. In this paper, the method of images is utilized to calculate the RCS of a hemispherical boss at 2.52 THz when the incident beam is Gaussian beam and the results are compared to that calculated when the incident is plane wave.