Abstract: Carbon nanotubes (CNTs)/PBO composites were synthesized by in-situ polymerization. Further more,the polymerization mechanism was discussed through some analyses contrasting with PBO and properties of the composites were studied also with TG and intensity measured. Analyses of samples from PBO and CNTs/PBO composites during polymerization in the same situation show that active groups on the carbon nanotube surface influence the polymerization greatly. Decreasing their addition and putting off their addition time could improve the polymerizing condition. It is found that CNTs/PBO composites have super thermal resistance like PBO and their tensile properties of fiber are enhanced by 40%~70% compared with PBO fiber produced in the same way.