A developed two-dimensional Finite Difference Time Domain (FDTD) method has been performed to investigate the optical bistability in a subwavelength metallic grating coated by nonlinear material. Different bistability loops have been shown to depend on parameters of the structure. The influences of two key parameters, thickness of nonlinear material and slit width of metallic grating, have been studied in detail. The effect of optical bistability in the structure is explained by Surface Plasmons (SPs) mode and resonant waveguide theory.