Petrochemical studies based mainly on major and trace element whole-rock XRF analyses have been made on the Horoman and the Nikanbetsu peridotite complexes, Hokkaido, Japan. Incompatible trace element abundances in peridotite samples were obtained by carefully examining the XRF lower limit of detection. The Horoman complex is divided into Lower and Upper Zones on the basis of geological and petrological features. There is a systematic variation between Mg# [100×Mg/(Mg+Fe)] and whole-rock chemical compositions in both zones. There are, however, several differences between the plagioclase lherzolite samples from the Upper Zone and those of the Lower Zone. First, the range of Mg# of the plagioclase lherzolite from the Upper Zone is more variable than that of the Lower Zone. Some samples from the Upper Zone have higher Mg# than the maximum values for plagioclase lherzolite from the Lower Zone. Such samples are significantly poor in Al₂O₃, CaO, TiO₂, Sr and Y. Second, there is a gap in TiO₂ content in the compositional trend for the plagioclase lherzolite from the Upper Zone. This observation enables the division of the plagioclase lherzolite into TiO₂-rich and-poor types. These results indicate that the compositional variation of the plagioclase lherzolite from the Upper Zone was caused by pervasive partial melting and variability in the extent of segregation of the partial melt. By contrast, the plagioclase lherzolite from the Lower Zone is inferred to have kept its original composition, which is consistent with petrographic evidence for no or very minor partial melting. Spinel lherzolite and harzburgite from the Horoman complex are rich in Sr and Ti relative to Zr and Y. Plagioclase lherzolite from the Upper Zone occurring close to the segregation vein has a character more enriched in melt component and was probably derived by addition of a melt component to the primitive mantle composition. The Nikanbetsu complex may have the same origin as the Horoman complex because of their similarity in major and trace compositions.