The mechanisms of photo-induced microstructures in an azobenzene polymer fi
lm are presented. They are based on the spatial periodic modulation of opti
cal intensity and the photoisomerization of azobenzene molecules with the m
ovement of main chains. Experiment and theory jointly point out the possibi
lity of photo-inducing desired spatial microstructures in an azobenzene org
anic polymer via adequate optical lattices and adequately polarized 'writin
g' beams.