This paper deals with the concept of waveguide characteristic impedance. It tries to prove that the conventional concept of rectangular waveguide characteristic impedance is not fully applicable. The article presents new analysis of the principle and method used to define waveguide characteristic impedance and gives a reasonable definition of rectangular waveguide characteristic impedance. The new concept is applied to long range circular waveguide. The author also presents in this paper his view concerning tolerance of the cross section of rectangular waveguide.