The photoisomerization of azobenzene derivative on the substrate requires a sufficient area to prevent stacking by the aggregation of azobenzene chromophores. In order to overcome such a close stocking of azobenzene chromophores, the azobenzene derivative (8-[4-[4'-octylphenyl]azo]phenoxy]decyl pyridinium salt, 8Az10py) with positively charged head group was synthesized and used after purification. The photoisomerization of 8Az10py was investigated with pressure-area isotherms. surface potential-area isotherm curves and BAM images of a 1:2 mixture of 8Az10Dy and cholesterol at thr air/water interface.