Abstract: The permeability and permittivity measurement technique of flat substrate materials at terahertz is presented in this paper. An improved phase-group delay method is researched for multi-value problem on dielectric constants measurement at high frequency based the traditional free-space theory. Oscillation relationship between the permeability and permitivity is used for eliminating the thickness-resonance problem. Error caused by calibration is suppressed based on fitting the material properties to polynomials. Examples of measuring air and a teflon substrate with a test system are given at last, proving that it is able to achieve an error precision of 2% from 0.17 THz to 0.22 THz.