An optical device with asymmetric transmission based on nonlinear silicon photonic crystal is designed. A common approach for producing nonreciprocal (asymmetric) transmission is to break the structural symmetry. For this purpose we utilize a photonic crystal waveguide that is asymmetrically side coupled to a cavity in both forward and backward directions. This asymmetric coupling together with the Kerr nonlinearity can provide the nonrecieprocal Transmission Rate of 46db in forward and backward directions. The photonic crystal waveguide is created by a removal a row of air holes from a two-dimensional Si slab photonic crystal with hexagonal lattice of constant a=477 nm and air holes radii of r=130 nm. The polarization of input light is assumed to be TE