The transmittance of defect mode in one dimension photonic crystals with nonlinear absorptional dielectric materials was deduced by the transfer matrix theory.The intensity enhancement factor of localized light in defect layer had also been obtained,which increases exponentially with the number of layers in a photonic crystal.The numerical calculation shows that a photonic crystal with saturable absorption dielectric materials in the defect layer has the property of bistablility.The threshold of input intensity to bring about bistablility is decided by parameters of refractivity?saturation intensity of defect layer and decreases exponentially with the number of layers in a photonic crystal.