Several important mid-infrared nonlinear optical crystals (ZnGeP2,GaSe,CdSe,GaAs,GaP,DAST) are introduced in this paper. The nonlinear optical properties have been displayed and compared. The recently progress of THz radiation based on nonlinear optical difference frequency generation(DFG) are summarized. Finally,the basic condition of DFG crystals for the excellent performance in THz radiation is given out.