Trans and cis forms of N-alkyl-4-styrylpyridinium derivatives (CnSP, n = 4, 8, 12, 16) were successfully synthesized and purified. The derivatives of styrylpyridinium cause photoisomerization when they illuminated with UV light. The pressure-area isotherms of CnSP and their derivative were studied on the effect of alkyl chain length. The photoisomerization of CnSP monolayers at the air/water interface was indirectly studied by measuring surface potential change with photoirradiation on the water surface. The characteristics of CnSP were furthermore studied with surface pressure-area isotherm, Brewster Angle Microscopy (BAM) and optical diffraction efficiency.