Pyrupc, generally ix1 the form of single crystals, is a common mineral in such mantle source rocks as kimherlites, and also serves as one of the important indicator minerals in placer prospecting for diamond.. It is found through recent studies that(1)pyrope is not a typical. dry phase nesosilicate mineral, with minor amounts of constitutional water contairxed in its lattice, and(2)the content of trace element Ni in pyrope sensitively indicates the formation temperature of pyrope. Ln this paper, the author has looked into content characteristics of some trace elements in pyrope such as Ni, Zn, Y and Zr, made a comparative study of formation temperature ranges of pyrvpes . from kimberlites of different ore potentialities, tentatively discussed the elationship between water content a.nd formation temperature of pyro'pe. and, on such a basis, dealt with the origin of constitutional water and its geological significan- ce. 25 typical coarse-crystalline pyropesamples were selectively collected from the No. 50 Icimberlite pipe in Liaoning Province. After flattening and polishing both faces, we performed proton probe trace element analysis and infrared spec- tral analysis. It has been found that pyrope from this rock body contains Ni 10-95 ppm, Zn 4-20 ppm, Ga 7-30 ppm, Sr 0-3 and Y 3-56 ppm. Accord- ing to geotlzermometer calculations of trace element Ni, 84% of pyropes were for- med above 1000℃, mainly iii the range of 1050-1150℃. The temperature-pre- ssure variations of the continental shield area suggest that these pyropes were formed in the diamond stable area of the upper mantle, thus indicating the exis- tence of diamond in the rock body. Kimberlite body in Shexian County of Hebei Province is a barren rock body, and analyses of samples from this rock body all demonstrate that the formation temperature is below 1000℃. Infrared spectral analysis of all samples shows that for six samples there exists an obvious absorption peak in the range of 3700一3500 cm-1，implying that these samples contain minor amour6ts of constitutional water. Calculations show that the temperature for the formation of hydrous pyrope varies from 1050 to 1250℃，higher than the stable boundary of 1000℃，hence demonstrating the close connection between the constitutional water and the formation of diamond.. Py- ropes containing constitutional water are all rich in Zr. The enrichment of incom- patible element suggests that the formation of constitutional wator might be cloesly related to the mantle metasomatism.