Abstract: The Vacuum chamber of the Separated Sector Cyclotron for Heavy IonResearch Facility Lanzhou to have been built is described. It is a large monolithicultra-high vacuum vessel with 10 m in diameter and 4.5 m high, weighing 65 tn.The total volume is about 100 m~3. There are 234 ports on it for various purpose. The mechanical analysis of the vacuum chamber is completed using a finiteelement method by means of the procedure SAP-5 C on a computer. Uddholm 316 L super-low carbon stainless steel made in Sweden was selectedas a structure material. Because of very big size, the monolithic vacuum chamber is divided intoeight bigger and several smaller pieces to fabricate in the factory. Then it istransported by a train and a truck to our institute. Finally it is welded as asingle vessel and machined in situ. In the interests of economy. The manualelectric arc welding is the best choice in our case. It is paid great attention tostrict technological process during welding. Owing to high requirements for ultra-high vacuum, all of tight welded seams are inspected and checked by liquidpenetration testing, X-ray examination and helium mass spectrometer detecting.All of the flanges are machined before welding as far as possible at the factory.Some big flanges are cut by special way in situ. The pressure of 8×10~(-6) Pa for sufficient beam transmission in the vacuumchamber is abtained in September of 1986.