The TM mode size, effective refractive index, effective pump area and coupling efficiency between pump and laser modes as function of Ti-stripe initial width W, diffusion temperature T and Ti-stripe initial thickness H in c-cut Tidiffused Nd:LiNbO3 waveguide laser have been theoretically investigated using variational method. The main features of the mode sizes in terms of these diffusion parameters were collected and compared with the published experimental results, a qualitative agreement has been obtained. The effective pump area, being proportional to threshold pump power, reveals weak dependences on both W and H in the considered respective ranges 4-16μm and 40-160 nm, and a strong dependence on T(950-1100℃); while the coupling efficiency relative to slope efficiency hardly changes (with values 0.82-0.85) with these parameters.