Laser-induced damage thresholds in KDP crystals at two polarization directions(i.e. parallel or pcrpendicular to the optical axis)have been measured at 1.06,0.532 and 0.355μm. It is found that the damage threshold sharply decreases as laser frequency increases. The experi-mental result shows that laser-induced damage in KDP crystals is mainly due to multiphoton ionization.