Photostriction-based all-optical actuation of silicon microcantilevers has been investigated through experimental characterization of structures fabricated on silicon-on-insulator substrates, and through numerical modeling and analysis of their semiconductor device and micromechanical characteristics. Since the pressure coefficient of the bandgap is negative in Si, photostriction-induced photoactuation in Si-based cantilevers was evident as upward mechanical defections (away from the substrate) in response to pulsed laser illumination on the cantilevers' top surface, which is in contrast to the downward deflections typical of photothermal effects. For the numerical modeling of photostriction induced effects, carrier lifetime and excess-carrier concentrations were determined from transient photoconductance measurements. The experimentally determined parameters were then employed to simulate carrier-density profiles across the modeled structure. The modeled cantilever deflections were found to be in excellent agreement with experimentally determined deflections. It is also shown that 100-μm-long Si cantilevers were deflected by up to 10 nm, and generated a force of 0.14 nN, when optically actuated by a 405-nm laser power density of 400 W/cm2.