Abstract: In order to quantitatively characterize the interphase of carbon fiber reinforced polymer composites, in situ peak force quantitative nano-mechanics (PF-QNM) technique was introduced. Its general principles, calibration method and applicability were analyzed. PF-QNM technique was used to measure the interphase dimensions and in situ modulus of each component of carbon fiber/poly-ether-ether-ketone (T300/PEEK), carbon fiber/poly-ether-sulfone (T300/PES) and carbon fiber/epoxy (T700/TR1219B). The results show that this technique can determine the width and elastic modulus of the interphase in nanoscale lateral resolution. The gradient increases of modulus exist in the three composites from the matrix region to carbon fiber region. The interphase thicknesses of T300/PEEK, T300/PES and T700/TR1219B are (69.3±7.9)nm,(101.3±10.2)nm and (48.4±5.4)nm, respectively. In this study, the interphase thickness of thermoset composites is smaller than that of thermoplastic composites. The statistical analysis shows that the average modulus of resin matrix of T300/PEEK, T300/PES and T700/TR1219B are 4.36 GPa, 4.96 GPa and 3.59 GPa, respectively, which is close to their elastic modulus in bulk materials.