The SmithPurcell radiation characteristic is analyzed from a train of line charges moving parallel to a sinusoidal metal reflection grating using an integral equation model. The numerical computations have been performed for short grating period with low-energy line charges and long grating period with high-energy line charges. The maximal radiated intensity is obtained at harmonics of the line charges modulating frequency. The bandwidth of the radiation becomes narrow, energy peaks are intensified. The normalized energy distribution produced by the high energy charges will have a narrow angular range emission peaks in the forward direction. The coherent radiation of frequency-locked will be generated gained from a train of line charges. The results show that the THz radiation will be produced for the reasonable parameters of line charge energy and grating.