The ultrathin films of polyaniline (PAni)/poly (styrenesulfonic acid)(PSSA) were fabricated via a novel self-assembling process by alternately immersing the substrates into dilute PAni solution in N-methylpyrrolidinone (NMP) and the aqueous solution of PSSA. The process was characterized by W-Vis absorption spectroscopy. It was found that the oxidation state of polyaniline in single monolayers was dependent on the thickness of the film. The self-assembling mechanism was based on the acid-base reaction between PAni and PSSA. The thickness of the films can be easily manipulated at nanometer scale by controlling the solution chemistry and recycling times. The resulting films are uniform and adhere strongly to the substrates.