Perturbation theory and analytic formulas are presented for modal analysis of step-index and graded-index multilayer planar waveguides. As examples, the dependence of guided modal gain/loss on structure parameters and polarization are discussed. The method is simpler and time saving than complex shooting method for solving complex eigenvalues, and can give accurate results for practical planar structures in semiconductor lasers and laser amplifiers. It can be appled to the analysis of influence of material gain/;pss coefficients of TE and TM modes and their modal gain.