Boron containing phenolic and cardanolic polymers were synthesized by transesterification and further polymerization with hexamethylenetetramine. Flammability and thermal stability of these polymers were evaluated by LOI and TGA, respectively, and compared with those of conventional phenol-formaldehyde and cardanol-formaldehyde resins. The polymers showed better flame retardancy and high thermal stability. The borate esters enhance the char formation decomposition reactions with the retention of the boric oxide surface coating during high temperature exposure of the phenolic mass and thus impart flame retardancy and thermal stability.