We describe a simple method to increase the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film by exposure to ultraviolet (UV) light in vacuum. Up to four order of conductivity improvement (from 10−3 to 50 S/cm) is achieved by irradiating PEDOT:PSS film with 254 nm ultraviolet (UV) light. Increased conductivity in UV treated PEDOT:PSS film is stable under ambient exposure. The mechanism for conductivity improvement is investigated by current-voltage measurement, atomic force microscopy, and absorption spectrum. Photo-cross-linking of PSS chains is determined as the reason for conductivity improvement. Our result demonstrates that UV treatment is capable of modifying the conductivity of PEDOT:PSS film independent of the process of film formation.