Abstract: Based on a series of electromagnetic field equations which are in line with the boundary conditions, the eigenequation of coaxial waveguide with outer slotted structure is derived using the fields matching method. When the radius of the inner conductor equals zero, the outer slotted coaxial waveguide is equivalent to an outer slotted cylindrical waveguide, and its eigenequation is derived as well. Numerical results reveal the propagation characteristics of both the coaxial waveguide and cylindrical waveguide with outer slotted structure. The deeper the slots are and the smaller the half angle of the slot gap is, the small the latent roots are.