As grown ZnO:Si nanocomposite films fabricated by thermal evaporation showed formation of an emission band in the photoluminescence (PL) spectra due to four prominent peaks viz 370, 410, 470 and 520nm all nearly of the same intensities overlapping each other. The best spectrum was obtained from 60nm thick samples. This study shows the contribution of film thickness controlled ZnO cluster density on the PL spectra. Also, the investigations reaffirm the role of interface in contributing the two blue peaks at 410nm and 470nm. Thus control of film thickness presents a method to tailor samples as per photoluminescence emission requirement.