Abstract:A canonical decomposition method for impedance tensor analysis in 3-D medium is put forward to cope with the problems in ordinary analysis of 3-D magnetotelluric impedance tensor. Based on 8 freedom degrees of MT impedance tensor Z, the method freely derives 8 independent parameters from Z, without any artificial restriction. These parameters may be used to describe electric properties of earth medium, and they have definite physical meanings and are interpreted very easy. Model medium analysis show that the parameters of special medium have their interrelations, so when they apply to natural medium they will bring the result which is some similar to the result of special medium, thus indicating dimensional characteristic of natural medium.