We have measured the surface and bulk damage thresholds as well as the multipulse accumulation effect of LN crystals. As a method of studying the damage mechanisim, nonlinear absorption process is investigated. It is found that the multiphoton absorption occurs both in bulk and surface damage processes, and is the origin of the damages. The direct reasons that result in visible damage are the stress explosion in the bulk and the thermal melting, thermal etching and plasma emission on the surface.