Unusual large resonant third-order nonlinear optical susceptibility X(3) of 6×10-3 esu was found in push-pull azobenzenes. This is essential to the onset of bistablity, Optical bistability has been demonstrated in glassy polymer film doped with such compo unds in Kretschmann configuration using 488nm Ar+ laser as light source. The electronic absorptive mechanism or mechanism of optically refractive index change of the observed bistablity is suggested by connecting it with the trans ois photoisomerization process of the investigating dyes.