Thin films of pyrene in polystyrene matrix have been prepared by spin coating technique.The concentration of polystyrene is kept constant to 1 wt.% while that of pyrene dopant varied in the range 2.30×10^-4-2.30×10^-1wt.%. Thickness of the films was found to depend upon  concentration of pyrene and varies from 90 to 782 nm. The results of X-ray diffraction analysis reveal the crystalline nature of the films. The optical properties were studied by absorption, excitation and Ì„uorescence spectroscopy. The band gap energy of pyrene in polymer films was calculated from absorption results. A transition from monomer to excimer is observed with thickness variation of the films. The structured part of the spectrum is assigned to the monomer emission while the broad emission band is attributed to well known pyrene excimer-like emission.