Raw multiwall carbon nanotubes (MWNTs) were first treated with strongly oxidated acid to form MWNTs-COOH and driven to a high dispersion quality in the dispersing medium via ultrasonic method. Then the MWNTs/poly (p-phenylene benzobisoxazole)(MWNTs-COOH/PBO) nanocomposites were prepared by an in situ polymerization technique. In this process, the morphological structure of MWNTs and MWNTs-COOH were investigated by XPS, FTIR, and TEM. The experimental results showed that the carboxyl group was introduced into the surface of nanotubes and the length of nanotubes was shortened. The images of SEM and AFM illustrated that the MWNTs-COOH was homogeneously dispersed in PBO matrix, and the DTA analysis indicated that the molecular weight of MWNTs-COOH/PBO was almost equal to that of PBO. Furthermore, the thermogravimetry results proved that the thermal property of MWNTs-COOH/PBO was more stable than that of PBO. Also, the knot strength and the tensile strength of MWNTs-COOH/PBO were 30% higher than that of PBO. In addition, the reaction route of the MWNTs-COOH and PBO oligomer was given according to the ATR-FTIR spectra of PBO polymer and MWNTs-COOH/PBO. (c) 2006 Wiley Periodicals, Inc.