Abstract: Carbon fibers (CF) were modified with different oligomeric silsesquioxane ( SSO) coatings to improvethe interfacial property of carbon fibers/ polyarylacetylene (CF/ PAA) . The interlaminar shear st rength ( ILSS) ofCF/ PAA was tested to make out the effect of t reatment . Atomic force microscopy (AFM) in the force modulationmode was adopted to study the cross-section surface of unidirectional CF/ PAA composites and the relative stiffnessof various phases , including CF , interphase and resin. Probability histogram and line dist ributing of CF/ PAA crosssection surface relative stiffness , obtained from the statistical analysis of the relative stiffness image , were used tocompare and study the interface characterizations of composites. The results show that the ILSS increases effectivelyand the effect s on interfacial characterizations are different af ter the CF surface was modified with different SSOcoatings owing to the different st ructures. Cage oligomeric silsesquioxane including large organic group s (Methacrylisobutyl-POSS) has the better t reatment result . AFM observations lead to the conclusion that an interfacial t ransi2tion layer with different morphology and stiffness appears in CF/ PAA composites af ter being t reated by the SSOcoatings of different st ructures. It can be inferred that the appearance of the t ransition layer may cont ribute to theimprovement on fibre/ mat rix adhesion.