A vacuum chuck is disclosed for holding and positioning wafers more stably and securely. The vacuum chuck includes a supporting assembly having a receiving groove and at least one first vacuum aperture defined in the receiving groove. A seal unit includes a seal ring bulging to form a vacuum trough. The seal ring is fixed in the receiving groove of the supporting assembly and has at least one second vacuum aperture communicating with the first vacuum aperture. A chuck connector fastened with the supporting assembly has at least one vacuum port and at least one vacuum orifice communicating with the vacuum port. At least one vacuum hose connects the first vacuum aperture, the second vacuum aperture with the vacuum orifice and the vacuum port of the chuck connector for evacuating the air of the vacuum trough to hold and position the wafer on the seal ring and the supporting assembly.