We have studied the changes in the orientational distribution functions of azobenzen-labeled polyamic acid monolayers before and after photoisomerization by measuring optical second-harmonic generation (SHG). From the surface SHG data, we determined the nonlinear-optical tensor components of the monolayers before and after the photo-isomerization. It is found that the point symmetry of the ate-pendant units in the monolayer changes from C-infinity upsilon to C-1 upsilon by the photo-isomerization.