Abstract: A flame retardant process is a kind of special combustion process where the burning rate is very low. On the basis of the basic principles of combustion the flame retardant mechanism of flame retardant materials is here studied. A physical and mathematical model are developed to describe the flame retardant charac terist ics of flame retardant materials. A flame retardant coefficient has been derived to express the flame retardance of these materisal, i.e.K=1/r K-ilame-retardint coeffic entr-burning rate of flame-retardant materialThe larger the flame retardant coefficient, the better is the flame retardant effect.