A novel stratification method (staircase approximation method) is presented to analyse dispersion characteristics of nonlinear slab sandwitched between linear substrate and linear cladding. Recursion formulas which are equivalent to extended transfer matrix method are used to calculate the dependence of propagation constant on power density. The method can be applied to non-Kerr type films as well as slab guide with graded-index. Computed results for typical examples are in good agreement with the exact results reported in literatures.