An improved loop test, which consists of twisting a loop in a nanoscale whisker adhered to a transmission electron microscopy (TEM) grid or other flat substrate and pulling the ends until the loop breaks, was developed for experimentally approaching the intrinsic strength of the whisker material. Alumina whiskers with diameters in the 82-320 nm range, as test targets, exhibited an average fracture strength of 39.1 GPa with a maximum of 48.8 ± 1.8 GPa, which is close to the theoretical prediction of ∼46 GPa. In situ TEM fracture analysis demonstrated the brittle fracture of alumina whiskers with thicknesses down to 82 nm. Conventional SEM/TEM imaging induced fracture of the whisker loops that had experienced high strain and a possible mechanism were discussed.