Abstract: The Yap Island Arc consists largely of metamorphic rocks and thus differs significantly from the most of island arcs of the western Pacific.It is crucial to study the origin and process of the metamorphic rocks in the Yap arc for delineating the tectinic setting of the Yap arc-trench system.In this study, the metamorphic rocks in the Yap arc were analyzed using an electron microprobe (EMP) to reasearch temperature-pressure conditions of the amphibolite rocks.Results of mineral assemblages in metamorphic rocks of Yap arc show that metamorphic facies of Yap metamorphic rocks are greenschist and amphibolite.Temperature-pressure conditions of the Yap amphibolite rocks were calculated by the thermobarometry and barometer on the basis of compositions of mineral pairs.The temperature-pressure results are 493.6~630℃/3.8~6kbar, and the facies series of which belongs to medium P/T series.The medium P/T series is the characteristic of common orogenic belts.Considering the results of mineral assemblages and temperature-pressure conditions, we concluded that the metamorphic rocks in the Yap arc were formed in the setting of a volcanic arcs.Metamorphic rocks in the Yap arc represent the metamorphic basements of Yap arc.The Yap amphibolite rocks were buried in the depth of 15~20 km and the exposure of metamorphic rocks is resulted from subduction erosion of the Caroline plate.Caroline Ridges and seamounts enhanced subduction erosion.The Yap arc basements underwent regional metamorphism, then the uplifted section of volcanic arc were eroded bythe subduction erotion.Eventually the metamorphic basements of Yap arc croped out the earth surface.