MongolOkhotsk Ocean（MOO） opend during the Silurian. Then the MOO plate subducted under the adjacent continents， forming a lots of magmatic rocks along the subduction zones， and accompanying with the deposition of thick flysch formations and the collage of sea mountains along HangaiHentelDaurian area. During Permian，the MOO closed at its most west part， forming the huge MongolOkhotsk gulf， and during this time many molasses formations unconformitily covered on the prePermian formations in Hangai area. From Triassic to Middle Jurassic，the MOO plate subducted under the Siberia plate and ChinaMongol continent， and formed very wide magmatic rock belt in North MongoliaTransBaikal area and Middle MongoliaEerguna massif.The MOO closed quickly and caused the terranes beside it collided strongly in the MiddleLate Jurassic，and finally formed the MongolOkhotsk Belt （MOB）. At the middle part of the MOB， there is a huge ductile shear zone called Erwendawaa ductile shear zone. The average attitudes of the foliations and lineations of this sheare zone are 327°/22° and 322°/19° respectively which indicate a toptothe SE thrusting. The zircon UPb ages of the deformational rock and later not deformational pegmatite from the ductile shear zone are 174±6Ma and163±4 respectively， which constrain the deformation age （about Middle to Late Jurassic） of this ductile shear zone. After the strong compression in the MiddleLate Jurassic，during Cretaceous， TransBaikal region suffered strong extension and formed many cretaceous extensional basins and metamorphic core complex， and accompanied with strong magmatism. The tectonic evolution of the MOB from Silurian to Permian was closely related with the northward subduction of the PaleoAsian Ocean. From Triassic to Early Jurassic， MOO plate subducted under the Siberia craton and ChinaMongol terrane normally；Then MOO closed quickly during the MiddleLate Jurassic，which was closely related to“East Asian multidirection convergent”；The Cretaceous magmatism， extensional basins and metamorphic core complexswas controlled by the delamination of the thickened lithosphere of the MOB.