Flame retardants are materials that inhibit or resist the spread of fire. Adding these kind of materials to woods, textiles, polymers etc. decreses their flamability. The effectiveness of flame-retardants is measured by the change in the critical oxygen concentration. We studied the flame-retardant effect of boron phosphate and how it changes the LOI of some resines. If the concentration of BPO4 in the mixture with photomer resines increases up to 50%, LOI of resines increases from 15-18 to 32-38%.