A Reversed Duckweed polymeric Langmuir-Blodgett (LB) film was characterized by Fourier transform infrared (FTIR) spectroscopy. It has a well-ordered structure which is similar to that of the monomer-octadecylacrylamide-LB film, which provides proof that the flexible hydrophilic network does not interfere the side-chain alignment. The phase transition behavior was studied by using temperature dependent FTIR and small-angle X-ray diffraction. The LB film shows a unique thermal transition behavior, which undergoes a three-phase transition processes : from a highly ordered structure transition into a melted disordered state through two Langmuir-Blodgett "liquid crystal" states.