Publication presents the new method of determining the cetane number of fuels used for feeding compression ignition engines. CN determination is done by spectrum analysis in spectro-photometer and by calculations of mathematical models. The check tests were carried out on more than ten derv samples. The obtained CN values were compared with the values of this parameter obtained according to norm PN-EN ISO 5165 in trustworthy laboratories. Subsequently the conclusions were drawn on the possibilities of applying the analyzer for determination of this parameter