Structure characteristics about activated carbon fibers (ACF) and polyimide (P84) doped ACF modified by HNO3 solution were studied to apply in mercury removal in coal-fired flue gases.The P84,which was always used in the non-woven fabric for bag filter,was intermingled with polyacrylonitrile-based ACF (PAN-ACF) in the weight ratio of 1:1 in order to make the doped ACF with P84 (doped-ACF-P84).Then the doped-ACF-P84 fibers were modified by HNO3 solution.The structure and morphology of doped-ACF-P84 were characterized and compared with those of ACF and doped-ACF-P84 modified by HNO3 solution.The results show that the modified doped-ACF-P84 fibers have almost the same pore structure and specific surface area comparing with the original one.However,contrasted with the original PAN-ACF,the doped-ACFP84 fibers modified by HNO3 solution have more oxygen-containing groups used for mercury removal.In particular,they have more lactone and carboxyl groups.