The effect of organophosphorus (OP) content on the flame retardancy and the flame retardancy mode of thermoplastic polyurethane (TPU) are reported. The limiting oxygen index (LOI) value for TPU increased from 23.1% to 24.6% and 33.5% by adding 20 wt% and 40 wt% of OP, respectively. UL-94 test showed V-2 rating for pure TPU, and V-1 rating and V-0 rating for the blends with 20 wt% and 40 wt% of OP, respectively. Potential reasons for the improved flame retardancy of TPU blends with increasing OP content were discussed based on the morphology of the char layers resulting from the combustion of TPU blends. The gas phase flame retardancy mode dominated in the composite with 20 wt% of OP by forming a loose, thin, and breakable porous char layer. The condensed phase charring mode dominated for the composite with 40 wt% of OP by forming a continuous, thicker, and denser char layer during combustion, which acted as a more effective barrier to reduce heat feedback to the polymer surface and thus resulted in the better flame retardancy. This study indicated for the first time that the content of flame retardant plays an important role in the flame retardancy mode of OP for TPU. (C) 2015 Published by Elsevier Ltd.