We discuss the effects of water on important igneous processes in the Archean, i.e., the komatiite magma genesis and the formation of the cratonic peridotites based on the experimental results on the effect of water on the melting relations of model mantle peridotite at 6.5 GPa. Liquidus temperature decreases by about 100℃ and the temperature interval to produce more than 60% partial melting reduces by about 50℃ in the 1〜2 wt% water bearing system at this pressure. Garnet is the first dissolved phase and the stability field of orthopyroxene expands in the conditions of the water content of about 5 wt% at 6.5 GPa. Aluminum undepleted komatiites (AUK) can be formed by melting at 200 km depth (6.5 GPa) in a hydrous mantle at significantly lower temperatures than under the dry condition. Aluminum depleted komatiites (ADK) also can be formed by hydrous melting at higher pressure. In addition, cratonic peridotites may be formed as residues of partial melting under various water contents in the Archean mantle. The stability of the cratonic mantle, tectosphere, may be explained by dehydration hardening by removal of water due to partial melting of the wet Archean mantle.