We investigated thermally- or optically-biased memristive switching in two-terminal micro devices based on vanadium dioxide (VO2) thin films. For the preparation of multi-level resistance switching, the device was kept at a specific temperature or an optical illumination power so that it fell into the thermal or optical hysteresis region of the device resistance during the switching. With the application of external current pulses, the device resistance decreased in a discrete manner showing multiple resistance levels, each of which was maintained as long as the temperature (or optical) bias excited the device. In particular, in the optically-biased case, the effect of the pulse-free interval between current pulses on the device resistance was also examined with respect to three intervals including 10, 15, and 30 s. It was observed that a longer pulse-free interval and higher optical bias reduced the rate of current-induced change in the device resistance. Finally, in order to explore a trend of grain resistance change in the VO2-based device, we carefully suggested a grain network model explaining a percolative transition in inhomogeneous VO2 film. (C) 2014 Elsevier B.V. All rights reserved.