In order to accurately calculate the femtosecond laser breakdown threshold of wide band-gap dielectrics using the photoionization and avalanche ionization theories, the applied range of these classical methods is demonstrated quantitatively by the comparison of the calculated results with the available experimental data. It is found that these standard theories to estimate laser ablation threshold are valid when the laser pulse duration is less than about 600 fs and the laser wavelength is more than around 400 nm. Besides, we demonstrate that the multi-photon ionization of wide band-gap dielectrics is the dominant breakdown mechanism when the laser wavelength is at the region of 0.4 mu m < lambda < 1.3 mu m. But the tunneling ionization mechanism performs a significant role when the laser wavelength is at the range of lambda < 1.3 mu m. Based on our calculation, we found that a valence band (VB) electron can absorb simultaneously the greatest number photons of ten in the multiphoton ionization process. Furthermore, it is revealed that the photoionization in dielectrics ionization process can provide seed electrons even at the pulse duration down to sub-10 fs. (C) 2012 Elsevier B.V. All rights reserved.