Carbon aerogels are potential super-thermal insulations for using in ultrahigh temperatures and in inert atmospheres.However, the mechanical properties and thermal insulating properties under ultra-high temperatures were not reported ever before.Here the compression and tensile strengths of carbon fiber-reinforced carbon aerogel composites at temperatures up to 1800 ℃ were measured under Argon atmospheres.The thermal insulating properties under Argon atmospheres for hot surface temperatures up to 2500 ℃ and under air up to 2100 ℃ were measured using graphite heating and oxygen-acetylene torch methods, respectively.The results show that the composites exhibit excellent high temperature stability, the compression strength for 10% strain at 1800 ℃ is 0.91 MPa, which is 78.4% higher than that at ambient temperature (0.51MPa).After single-side heated at 2500 ℃ for approximately 1500s under Argon, the shrinkage at thickness direction is 4.92% for a 10 mm thick composite plate.When heated to 2000 ℃ at Argon atmosphere, the cold surface temperature of the aerogel composite (1025 ℃) is 276 ℃ lower than a rigid carbon fiber felt (1301 ℃) with the same 40 mm thickness.