Petrology of peridotite xenoliths derived from the upper mantle of the Japan arcs is reviewed in order to understand sub-arc mantle (or wedge mantle) materials and processes. All mantle xenoliths of the Japan arcs are included by volcanics (alkali basalts and arc basalt to dacite) younger than 17 Ma. The mantle peridotite is mostly Iherzolite and rarely harzburgite with the Cr# (=Cr/[Cr+Al] atomic ratio) of spinel less than 0.7, and the Na₂O content of clinopyroxene is lower then in continental and oceanic hotspot peridotites and is sometimes as low as that in abyssal peridotites and is sometimes as low as that in abyssal peridotites. The Japan-arc mantle peridotites are thus almost similar to abyssal peridotites in terms of major-element mineral chemistry. They can be, however, distinguished from abyssal peridotites by Ti-Zr ratio and other trace-element geo chemistry of clinopyroxene. They are most probably restites after influx-assisted melting. The Japan-arc peridotites have been intruded and metasomatized to various extent by magmas and related fluids of arc origin or of origin of intra-plate plume responsible for opening of the Sea of Japan.