The effects of swift heavy ions on the structure, surface morphology and dielectric behaviour of barium titanate (BaTiO3, known as BTO) thin films have been investigated by irradiating the films with 100 MeV Au+8 ions at an ion fluence of 1 x 10(12) ions cm(-2). BTO thin films were deposited on Pt/Si substrate by pulsed laser deposition technique. The structure of the film was crystalline with tetragonal phase. Surface topography was studied by atomic force microscope. The dielectric properties of BTO have been studied using surface plasmon resonance (SPR) technique in Otto configuration. SPR is a highly sensitive technique for investigating the dielectric and optical properties of dielectrics. Gold-coated glass prisms have been utilized to excite the surface plasmon waves (SPWs) at the metal-air interface. When BTO thin film is integrated with the surface of Au thin film, a significant shift in the SPR curve has been observed due to change in the propagation constant of SPW at metal-dielectric interface. The optical and dielectric parameters of BTO thin film were obtained by fitting the experimental SPR data with the Fresnel's equations. The observed increase in the dielectric constant of BTO thin film after irradiation may be attributed to packing density.