The work titled Evaluation of flammability of chosen polymers by oxygen number method summarizes the findings about burning of polymer materials. With focusing on polystyrene flammability. It describes polymers partition, particularly their chemical composition and inferential properties. The work deals with polystyrene plastics - namely its divestment, flammability, their burning products as well as forms of their retardation. The aim of the work is to valorize the method of experiment used for assesment of the chosen fire-technical characteristics – oxygen index. It simultaneously describes more methods used for assesment of the fire-technical characteristics. In order to evaluate the flamability of five polystyrenes used in building, the work uses STN EN ISO 4589-2 2001. In addition to evaluating the flamability, It also compares the given results and their changes depending on many different physical aspects.