The adsorption of chlorine on a Pt(111) surface at 90 K has been studied using scanning tunneling microscopy (STM) in ultra-high vacuum environments. The adsorbed chlorine atoms give rise to two different ordered structures, (3${ imes}$3)-Cl and c(4${ imes}$2)- Cl, depending on the coverage. It has been determined from the STM image that the absolute coverage of (3${ imes}$3)-Cl structure is 0.44, which is not in agreement with coverage calibrated by a low energy electron diffraction technique together with an Auger electron spectrometer and a thermal desorption spectrometer. The Cl atoms bound to on-top sites at the Pt(111)(3${ imes}$3)-Cl surface appeared to effectively perturb the density of states of Pt atoms, as compared with that bound to bridging sites. The other ordered structure, c(4${ imes}$2)-Cl, with small domain sizes, consists of both on-top and bridge-bonded species with a saturation coverage of 0.5.