Abstract: The fabrication, microstructure, compressive property and thermal performances of the phenolic foam reinforced with carboxyl carbon nanotubes(CNT-COOH) were studied. The reinforcement mechanism of CNT-COOH was investigated by analyzing FTIR and detailed failure behavior of 0.05%(mass fraction) CNT-COOH/phenolic foam composite and phenolic foam under different compressive strains. It is found that serving as the site of heterogeneous nucleation, CNT-COOH increases the cell density and decreases the cell size of the produced foams, and that CNT-COOH reinforced foams are evidently stiffer and stronger than the corresponding neat system as the increment of CNT-COOH content. FTIR reveals that curing reaction of CNT-COOH and phenolic resin may not exist. SEM analysis of the composite foams in different compressive strains reveals that adding CNT-COOH located in the cell walls could endure a certain load through the interface between CNT-COOH and phenolic resin, thereby enhancing the compressive strength of CNT-COOH/phenolic foam composites. In addition, thermogravimetric analysis (TGA) and vertical burning method show that CNT-COOH as stabilizer, reduces thermal degradation rate, leading to slightly higher thermal stability and resistance to flame of the foam composites.