Phenolic aerogel/carbon fiber composites with different microstructures have been prepared by sol-gel polymerization of phenolic resin and curing agent. The particle size and pore size of phenolic aerogel in the composites were controlled by changing the amount of curing agent. It was found that the particle size of the aerogels could be decreased from 368 nm to 54 nm with the decrease of the curing agent amount

corresponding to the decrease of average pore size from 5 μm to 230 nm. Furthermore

the properties of composites were strongly depended on the microstructure of aerogel

in which the mechanical strength and ablation property increased while the thermal conductivity decreased with nanoparticle size decreasing. The composites with the minimized particle size had a low density of 0.27 g·cm-3

a high bending strength of 8.9 MPa and a low room-temperature thermal conductivity of 0.065 W·m-1·K-1. Under the oxyacetylene ablation conditions of 2000℃ for 30 s

its mass/linear ablation rate were 0.0081 g·s-1 and 0.0204 mm·s-1

respectively. By regulating the nanostructure of material

it can effectively improve the mechanics

heat insulation and ablation performance of the material to meet the needs of high performance thermal protection applications.