This paper investigates the effects of brominated and halogen free fire retardants on the thermal degradation, flammability and fire performance of glass fibre reinforced PA66 (PA66/GF). The base formulation contains PA66 reinforced with 30% glass fibre by weight. This base formulation was modified with either brominated polystyrene/antimony trioxide (Milebrome 7010/Sb2O3) or phosphorous based flame retardants (Exolit OP1311 and Exolit OP1311 with/without Zinc Stannate or Zinc Borate). Exolit OP1311 is a mixture of aluminium diethyl phosphinate (Alpi) and Melamine Polyphosphate (MPP). The thermal stability and char residue are characterized by Thermogravimetric Analysis (TGA) whereas flammability and burning behaviours by the cone calorimeter. TGA results indicate that all fire retardants promote the degradation of PA66 due to the decomposition of fire retardants (FRs). The cone calorimeter results show that brominated polystyrene has little effect on ignition or mass loss but reduces the heat release rate by a factor of 3 due to its strong action in the gaseous phase manifested by a 70% reduction in the effective heat of combustion compared to the base formulation. In comparison, all Halogen Free Fire Retardants (HFFRs) delay ignition and reduce both the mass loss and heat release considerably. The effects of HFFRs are mainly in the solid phase as they intumesce upon heating and form a thick surface layer which protects the unpyrolysed material underneath. In terms of production of smoke, the formulation containing brominated polystyrene has much higher smoke production rate than other formulations but similar smoke yield to the HFFRs. Brominated polystyrene also has much higher CO production rate and yield than other formulations and in certain case 20 times more that of the base formulation.