The structure, electronic properties, and chemical reactivity of pyrene and its three monochlorinated derivatives have been theoretically studied by using density functional theory (DFT). Several methods and basis sets are tested for the optimization of the geometrical structure and the B3LYP/6-311G** method/basis set is found to be the most suitable in predicting the heat of formation of pyrene and the minimum energy optimized structure of pyrene and its monochlorinated derivatives. The geometrical parameters and the heat of formation of pyrene are in good agreement with available experimental data. The chemical reactivity and stability of the four compounds are investigated. The computed results agree well with experimental results of monochlorination of the pyrene molecule. The relative stability of the monochlorinated pyrene compounds is in the order: 1-chloropyrene > 4-chloropyrene > 2-chloropyrene.