We discussed the nonlinear effects of ponderomotive force, which is exerted by powerful LH waves with large electrical fields (|E|～1.5 kV/cm). An obvious nonlinear nonhomogeneity of density in the direction parallel to B0 has been found. It might be serious enough to influence the propagation of the long waves (such as waves with N‖～1). As for the direction perpendicular to B0,if there is reflection inside the plasma (for example, from the region close to the limiter of Tokamak), a series of density cavities might appear along the path of LH wave transmission, and the picture of plasma displacement is no longer suitable.