In order to investigate a constitutive law associated with shear band in amorphous glassy polymers, polycarbonate specimens were tested under forward and backward loadings in simple shear at room temperature. It was shown that (1) the shape of shear stress-strain curve was decided by the formation, lengthening and widening of the shear band at both loading paths, (2) the stress drop at yielding was very sensitive to the lengthening of shear band and (3) once the shear band formed, the yield drop did not occur at backward loading.