Abstract: An eddy current test theory was proposed to overcome the limitation of the current test technology, where only one parameter (magnetic permeability or electric conductivity) was obtained and then some intrinsic features of the conductor were analyzed via this parameter. In this new test theory, by introducing parameter M, a mathematic model was established based on the Ohm law and the cylinder form conductor magnetic field permeability formula. An absolutely eddy current sensor was designed based on this theory. Experiments showed the feasibility of the theory.