We report here a new technique of lithography in thin polyaniline film formed at the air-water interface, by simultaneous chemical and photochemical polymerization of aniline. A solution of aniline in acidic KBrO3/KBr polymerized to form thin film at the air-water interface. When the polymerization was carried out in the presence of light at the interface, a macroscopic pattern could be seen formed on the film in the shape and size of the incident light beam. A desirable shape of the pattern could be achieved by using appropriate optical mask. The lithographic pattern could be imprinted using incident light of wavelength in the range of 320-650 nm. Observation under optical microscope revealed that nucleation pattern of polymer growth was absent in the area of the polymer film grown in the presence of light that otherwise was present in the film formed in dark. FTIR spectra of polymer films grown in light and dark were the same.