By use of optical second harmonic generation (SHG), an irreversible phase transition from a disordered to an ordered structure is observed in thin pyridine (C5H5N) films dosed on a Ag(111) surface under ultrahigh vacuum. This phase transition occurs when the surface temperature is raised above 125 K. An analytical model of SHG from the annealed pyridine film including optical interference is presented. It is most likely that the annealed films are composed of crystallites randomly oriented on the surface plane with an average length scale comparable to or larger than half a micrometer.