Based on the linearity of ablated area and laser pulse energy, the threshold fluence of sapphire is determined accurately in this paper. Meanwhile the dependences of F-th on pulse duration (tau < 1 ps) with laser at 400 and 800 nm are presented, respectively. It is found that the experiment results agree well with the theoretical calculations based on avalanche model. And we discuss the photon absorption mechanism of the conduction band electrons in Al2O3 single crystal. (C) 2003 Elsevier B.V. All rights reserved.