Phenolic foams reinforced with pristine and functionalized multiwalled carbon nanotubes (MWCNTs) were fabricated to develop fire-resistant materials with improved mechanical properties. The influences of the contents of carboxyl multi-walled carbon nanotubes (MWCNTs-COOH) and of MWCNTs types on the compressive properties of the composite foams were investigated. The microstructure and detailed failure behavior of MWCNTs/phenolic composite foams were studied using scanning electron microscopy (SEM) and in situ quasistatic compression inside SEM, respectively. In addition, thermal performances were evaluated by thermogravimetric analysis (TGA) and vertical burning method. It is found that as heterogeneous nucleation agents, MWCNTs increase cell density and decrease cell size of the produced foams, and that as reinforcements located in cell walls, MWCNTs impart high strength and stiffness to brittle foams. Moreover, MWCNTs reinforced foams have higher thermal stability than raw foams and exhibit similar excellent resistance to flame, confirming the effectiveness of MWCNTs as stabilizers. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1479-1488, 2013