Melamine polyphosphate and thermal-plastic polyurethane (TPU)-encapsulated solid acid were applied for flame retardant glass fibers reinforced polyamide 6 (GFPA6). The introduction of TPU would change the interfacial property between glass fibers (GFs) and polyamide 6 (PA6), weakening the "candlewick effects" of GFs in PA6. Serving as a synergist, solid acid containing sulfur (CAS) played the role of a strong acid source, which could promote the system to form much more condensed and closed char layers. Macromolecular charring agent, TPU, was able to accelerate the charring process. In addition, TPU encapsulating on the unstable solid acid could isolate CAS from PA6 resin, preventing the chemical interaction between them, which would cause the degradation of material. This established technology provided an effective approach to prepare halogen-free flame retardant GFPA6 with UL94-1.6 mm V0 rating and good mechanical performance, showing a promise in the future commercial application. (c) 2007 Wiley Periodicals, Inc.