The eigenvalue and cut-off equations of TM modes in diffused c-out LiNbO3 waveguides have been derived using ray-optics method. For diffused waveguides with exponential and Gaussian index profiles effective refractive index as a function of the diffusion depth, and cut-off diffusion depth curves as a function of the waveguide surface index increment for some lower order modes have been plotted using these eigenvalue and cut-off equations, which can be used in measurement and fabrication of diffused c-cut LiNbO3 waveguides.