Hydroxyapatite (HAP) nanorods were synthesized via a facile hydrothermal method, which were used as nanoadditives to prepare the flame-retardant microcrystalline cellulose (MCC) composite aerogels. Flame-retardant and thermal properties of MCC/HAP composite aerogels were evaluated. When tests were performed at room temperature, the composite aerogels exhibited enhanced thermal stability and low thermal conductivity but more rapid thermal dynamic transfer rate during heating and thermal dissipation rate during cooling compared to pure MCC aerogel. The MCC aerogel containing 50 wt % HAP yielded a reduction of 93.7% in peak heat release rate (PHRR), and the smoldering occurred when exposed to a flame or the cone heater. The remarkable improvement in the flame-retardant properties of MCC/HAP should be attributed to these possible mechanisms: (i) the increased thermal dynamic transfer performance during heating has an adverse effect on the increment in time to ignition and time to PHRR; (ii) the nonflammable H...