This disclosure relates to a process for making flame retardant fibers comprising the steps of combining a flame retardant masterbatch with a polymer to form a polymer composition and forming the flame retardant fibers from the polymer composition. The masterbatch comprises greater than 10 wt % of a nitrogen compound having a melting point less than 400° C.; and a masterbatch polymer, with the weight percentages based on the total weight of the masterbatch. The flame retardant fibers pass the flame retardancy standards of ASTM D6413 (current year).