Abstract: THz wave radiation induced by femtosecond pulse in periodically poled nonlinear crystal is theoretically studied based on optical rectification. Using the principle of the antenna radiation and the theory of the grating diffraction, the study focuses on the distribution of the field of the frequency domain and the field of the time domain. The center frequency, the width of the spectrum and the electronic field of the THz wave are discussed and analyzed varying with the radiation angle. The result of the study shows that the bandwidth of the THz wave is inversly proportional to the length of the crystal and the number of the grating, and that the electronic field varying with the radiation angle is quasi-harmonic.