The optical nonlinear properties of an effective anisotropic slab made of metal-dielectric lamellar gratings with a small grating-period-to-wavelength ratio ∧/λ→O studied.It is shown that when a transverse magnetic electromagnetic wave obliquely illuminates such a slab,whose effective normal permittivity is much smaller than its tangential one,the normal wave vector inside the effective medium is extremely sensitive to the fluctuation of the tangential effective permittivity.Based on this characteristic,obvious nonlinear modifications of the phase delay and step-like transmission are obtained by a moderate input intensity even if the thickness of the slab structure is only less than λ/6.