The interphase size and properties of carbon fiber reinforced poly(ether-ether-ketone) composites (T300/PEEK) were quantitatively characterized using Peak Force Quantitative Nano-Mechanics (PF-QNM) with atomic force microscopy (AFM). The elastic modulus maps of the interfacial region were obtained, and the interphase dimension was determined based on the elastic modulus profile. The interphase thickness of T300/PEEK under coupled ultraviolet (UV) and hydro-thermal degradation decreased from 70.1 ± 8.6 to 18.3 ± 1.8 nm after 1560 h of exposure. The shrinkage of interphase size was attributed to the embrittlement of PEEK after crosslinking reactions induced by UV exposure. This technique shows a great potential in the quantitative measurement of nanomechanics relevant in polymer and polymer composites, where the structure and properties of the interphase are of special interest.