Starting with rate equation and taking the spatial distributions of pump and laser modes into account,we have derived the general expressions of the threshold pump power and slope efficiency for Nd:MgO:LiNbO 3 and Ti diffused Er:LiNbO 3 waveguide lasers.Then,by assuming Gaussian/Hermite Gaussian or Gaussian/two half Gaussian mode intensity distributions in the directions of width/depth of the waveguide,their theoretical values were calculated on the basis of experimental condition and most of them agree with or close to the measured results.