Terahertz (THz) wave is the wave which is between in the microwave and infraredrey.It is easy to penetrate the chinaware,fat,carbon board,drapery,plastic and so on.So it can be used to detect the gap and the lacuna of the conduit.The technology of THz can be used to study the absorption coefficient,refractive index,permittivity and frequency shift of the material.There are many ways to generate the THz wave including photoconductive switch,optical rectification,TWT and so on.