Disclosed is an optical film that is manufactured by using longitudinal and uniaxial stretching of an unstretched cycloolefin copolymer film and has an in-plane retardation of 100 to 150 nm and a thickness retardation of 0 to −30 nm at a wavelength of 550 nm, a polarizing plate and a liquid crystal display including the same, and a method of manufacturing the optical film. The method includes longitudinally and uniaxially stretching the unstretched film while a ratio of a width to a length of a stretched portion of the film is controlled.