In this paper, the ultramafic rocks distributed within underthrust collision orogenic belts are divided into two groups, namely, ophiolites and ultramatic rocks (UM) of volcanic arc type; and the latter is subdivided into Yidun type,Alaska type and peridotitediorite type. If both ophiolites and UM of volcanic arc type appear in parallel distribution in the same orogen, and are emplaced in nearly the same period, they may form a pair of ultramafic belts. The ophiolites represent the remaining traces of a closed oceanic basin，while the UM，which are usually emp,laced in the late stage of the orogenic movement, indicate an island arc or active continental marginal setting on the obducted side of the plate above the subduction zone. Recent work in the Hengduan Mts region has led the authors to conclude that there are a pair of ultramafic belts in the region; the eastern belt is composed of ophiolites(including Yushu一Litang belt and Ailaoshan belt)，while the western belt aonists of UM of volcanic arc type (including the three subtypes)·These two belts are roughly parallel in distribution and both were emplaced in almost the same period, t. e.，Late Triassic. As it is known that paired UM belts probably exist in Venezuela, the Appalachians, western U. S.,Ural and the maritime regions of U.S. S. R.，Japan and the south Island of New Zealand. Moreover, there are also clues for the existence of paired UM belts in Yinjiang and Qilian Mts region in China. Therefore, the authors think that the distribution of paired UM belts might be of worldwide significance. With the exception of some cases, if the subduction of plate occurs within an orogen paired UM belts might be formed therein. The authors also noted that the metamorphic belt might be related to the paired ultramafic belts. However, compared with the former, the paired ultramafic belts are probably more implicative.