Abstract: Based on the principle of dielectric constant of materials measurement by free space method,the authors measured,under room temperature,the complex dielectric constant of coals which are different types and from different areas,and studied the dependence of moisture and the dielectric constant of coals at terahertz (THz) band electromag- netic radiation from 100 GHz to 110 GHz. The study provides a theoretical and data foundation for the research and de- velopment of the borehole radial crack detection technology which is based on the THz transmission imaging in order to prevent and control coal mine water disasters. This experiment measured the complex dielectric constant and calculated the loss tangent of coals under different moisture conditions at 100,105 and 110 GHz. Through the analysis of experi- ment data,under room temperature,it concludes that both the real part and imaginary part of dielectric constant of coal increase with moisture increase,and the loss tangent also presents an increasing trend with moisture increase at tera- hertz band electromagnetic radiation.