This study demonstrates the microscale and nanoscale mechanical properties of a multiwalled carbon nanotubes/polyether ether ketone (MWCNTs/PEEK) composite utilizing a novel in situ testing method. Nanoscale testing specimens of a composite of PEEK thin film dispersed with 6.5 wt% MWCNTs were successfully prepared using focused ion beam (FIB) and investigated by a tensile testing device accommodated inside a field emission scanning electron microscope. The average tensile strength of the nanocomposite was measured to be 388.66 MPa. The average Young's modulus measured at 6.52 GPa demonstrated a 23% increase compared to that of the bulk specimen, suggesting a size-dependent mechanism that limits the plastic deformation of the microscale specimens possibly due to the limited defects distribution and motion constraint of the polymer chain in the small specimen and the amorphous regions of the PEEK matrix. POLYM. ENG. SCI., 59:1209-1214 2019. (c) 2019 Society of Plastics Engineers