The problem of bending of orthotropic rectangular plates with clamped edges on elastic foundation may be reduced to the following differential equation and boundary conditions (?4w)/(?x4)+2λ(?4w)/(?x2?y2)+(?4w)/(?y4)+kw=q/D. w=0, (?w)/(?x)=0 at x=±a, w=0, (?w)/(?y)=0, at y=±b. In the case of isotropic plates, λ = 1. In this paper a perturbation method is proposed for the solution of this problem fay expanding w in power series of λ: w=w0+w1λ+w2λ2+……. It is proved that this series is convergent when -1 ≤λ≤1.