Two models to calculate the irradiance threshold for laser-induced breakdown for water have been introduced. Both of the models have been incorporated into a computer code and code results compared to experimentally measured irradiance threshold for breakdown of pure water by nanosecond, picosecond and femtosecond laser pulses in the visible and near-infrared. The result shows that the second one has a wider range than first one to agree with the experiment data. For nanosecond laser pulses, the generation of free electrons in distilled water is initiated by multiphoton ionization but then dominated by cascade ionization. For shorter laser pulses, multiphoton ionization gains ever more importance, and collision and recombination losses during breakdown diminish. The threshold near infrared is more depend on the initial electrons.