The mechanical resonance of laterally grown silicon nanowires measured by an optical interferometric technique is reported. The lengths and diameters of the nanowires ranged from L=2 to 20 μm and D=39 to 400 nm, respectively. The wires showed resonant frequencies in the f0=1–12 MHz range and resonant quality factors Q at low pressure ranging from Q=5000 to Q=25 000. The dependence of resonant frequency on the ratio of diameter to length squared, D/L2, yielded a ratio of . Assuming a density of ρ=2330 kg/m3, this experimental result yields an experimental Young modulus of E=205±10 GPa, consistent with that of a bulk silicon. As the wires were cooled from T=270 K to T=77 K, a 0.35% increase of resonant frequency was observed. This increase of resonant frequency with cooling resulted from a change in Young’s modulus and from the thermal contraction of silicon. The quality factor did not vary significantly from P=10-4 to 102 Torr, suggesting that viscous damping does not dominate the dissipative processes in this pressure range. Although viscous damping became important above P=102 Torr, relatively high quality factors of Q=7000 were still observed at atmos- pheric pressure.