The nonlinear evolution of filamentation instability has been investigated in an inhomogeneous ionosphere. It is seen that a plane wave of uniform intensity becomes unstable and gets filamented in the presence of transverse density fluctuations in the medium. The amplitude of the filaments increases with altitude and increasing incident wave power, behaving like an Airy's function. The horizontal scale size of the filaments is equal to the horizontal wave number of perturbation. The spatial growth length of instability is of the order of density scale length.