A new model is proposed which greatly improves the accuracy in predicting the ablation depth and, for the first time. can predict the flat-bottom crater shape for wide bandgap materials ablated by a femtosecond laser pulse. The model calculates the transient distributions of free electron density and free electron temperature. The quantum treatment is employed to account for the specific heat and the relaxation time for free electrons. The temporal and spatial dependent optical properties of the dense plasma are considered. The predicted threshold fluence and ablation depth for fused silica are in agreement with published experimental data. (C) 2004 Elsevier Ltd. All rights reserved.