Smith-Purcell radiation is emitted when an electron passes above the surface of a metallic grating. Its mechanism can be explained with Huygens' principle by the radiation of a moving oscillating dipole, which is formed by the moving charge and its image in the metallic grating. Here, an alternative way is presented to enhance the THz Smith-Purcell radiation. By drilling a hole in the fins of a grating as an effective electron channel, the oscillation dipole happens in two dimensions here, instead of one dimension. As a result, the Smith-Purcell radiation power is ten times more than the case in which the electron passes very close to the grating surface. This method is expected to improve the efficiency of the devices which are based on the Smith-Purcell radiation.