Analysis of kinetic mechanisms responsible for the combustion enhancement in HCCI engine using the natural gas as a fuel through the addition of small amounts of chemically active species such as atomic oxygen and singlet delta oxygen that can be produced due to photodissociation or laser-induced excitation of O-2 molecules is conducted. The simulation, based on zero-dimensional single-zone thermochemical model, shows that the excitation of O-2 molecules to the a(1)Delta(g) state is more effective in the advance of ignition timing and combustion performance than the production of atomic oxygen. However, in order to obtain an advance in HCCI combustion via photodissociation or laser-induced excitation of O-2 molecules, one needs to diminish the initial temperature of the mixture in the cylinder or to decrease the compression ratio.