Abstract Mechanical property evaluation of nanometer-sized structures is necessary to help design reliable MEMS/NEMS devices. Most material properties are known to exhibit dependence on specimen size and such properties of nanoscale structures are not well characterized. Silicon and SiO 2 nanometer-scale beams (nanobeams) with a 6 μm length and widths ranging from 200 to 600 nm were fabricated using lithography-based techniques. A quasi-static bending test technique for these nanobeams was developed using an atomic force microscope (AFM). This technique was used to evaluate elastic modulus, bending strength and estimate fracture toughness of the beams and beam materials. The beams failed in a linear elastic and brittle manner. Results indicate that elastic modulus and fracture toughness values are comparable to bulk values, whereas bending strength appears to be much higher for these nanobeams than for larger scale specimens, thus revealing a size effect. We also report results from monotonic cyclic loading tests of the nanobeams that reveal nanoscale fatigue performance of the beam materials. SEM observations of the fracture surfaces suggest cleavage as the fracture mechanism for both Si and SiO 2 beams.