The taper waveguide is useful to make short the length of matching device between different characteristic impedances.The taper profile may produce the propagation mode transformation. So. it is remarkable to design the profile of taper waveguide. For multi-mode analysis, the method based on transmission line model is not so applicable. In this paper, we discuss the technique of multi-mode analysis of circular taper waveguide using finite elementmethod(FEM) first. Next, the results of analysis by the program we developed may be shown. As the result. using axis-symmetric triangular ring elements shows a peak of the absolute value of complex mode amplitude of transverse magnetic field in a taper waveguide. That mode cannot propagate through the uniform waveguide with same cross section. This show the affection by the taper shape of waveguide.