The proton microprobe and the quantitative micro-PIXE analysis technique are used to determine trace element composition and distribution in minerals of mantle peridotite, therefore, additional useful information is provided on the study of upper mantle mineral phase transformation. An example reported here is the study on the geochemical behavior of trace elements in minerals of Suoluoshu mantle peridotite from Shandong province, which was in the process of mineral phase transformation from spinel to garnet. The study shows that trace element composition and distribution display obvious change in minerals while the mineral phase is transformed from spinel to garnet. Most of the trace elements analyzed reside in clinopyroxene and spinel. However, garnet is nearly poor in all of these elements. This phenomenon is associated with the infiltration of solution and the existence of micrograined inclusions in minerals in the process of mantle metasomatism.