Abstract: An atom force microscopy in force modulation mode was adopted to study the cross-section surface of unidirectional carbon fiber/polyarylacetylene (CF/PAA) composites, and the shape, distribution and relative stiffness of various phases were obtained. The probability histogram of surface relative stiffness, obtained by the statistical analysis of the relative stiffness image, was used to compare and study the interphase characters of the composites modified by different methods. The results show that obvious interphase can not be seen in untreated composites, and the structure of the interphase area becomes more complex after oxidation treatment due to the change of stiffness. More perfect interphase can be obtained and a transitional area with moderate modulus can be established between fiber and matrix after the treatment of CF by a combined method of oxidation and coating, so that the mecha-nical properties of the interface are improved obviously.