The interface quality of Carbon Fiber Reinforced Polymer (CFRP) composites is critical to the structural integrity of composites. The environment in which composites are used also has significant effects on their performance. The objective of this work is to characterize CFRP composites, their interface and the effect of hygrothermal treatments using Atomic Force Microscope (AFM). The cross-sections of three CFRP epoxy composites were polished and investigated with AFM. AFM height images showed fibers higher than matrix caused by polishing. Hygroscopic treatment progressively reduced the height difference between fiber and matrix indicating the swelling of matrix by absorption of moisture. Thermal treatment increased the height difference, believed to be caused by matrix shrinkage resulted from additional cure under elevated temperature. The sheath-core structure of AS4 and T700 fibers manifested itself in both height and phase AFM images. AFM phase images revealed fiber/matrix interface susceptible to hygrothermal environments, implying that degradation and failures likely initiate at the interface. © 2006.