Thin films of perylene in polystyrene matrix have been prepared by spin coating technique. The concentration of polystyrene is kept constant to 1 wt% while that of perylene dopant varied in the range 1×10^-5-1×10^-1 wt%. The thickness of the films depends upon the concentration of perylene and varies from 45.48 to 582.16 nm. The structural aspects of the films were studied by recording the X-ray diffractogram of the films. The results of X-ray diffraction analysis reveal the crystalline nature of the film. The optical properties were studied by recording absorption, excitation and fluorescence spectra. The band gap energy of perylene in polymer films was calculated from absorption curves. The fluorescence spectra of the films show monomeric as well as dimeric emission bands. The films with lower concentration of perylene show monomeric emission similar to that of dilute solution, while the films with higher concentration of perylene exhibit structure-less broad emission. These emission bands appeared at 509 and 525 nm are attributed as arising from dimmer, formed in the ground-state by parallel orientational alignment of neighbouring molecules. The additional bands observed in the blue region of the absorption and excitation spectra of the films containing higher concentration of perylene confirm the formation of ground-state dimer.