The design and a prototype of the vacuum system of a low-emittance 3 GeV synchrotron light source, the Taiwan photon source (TPS) (with circumference of 518.4 m), are described. The TPS vacuum system has low-outgassing aluminum beam ducts, a low-impedance structure, an oil-less pumping system, and oil-less fabrication. Little dust, a stable mechanical structure, and highly reliable components are achieved for this vacuum system. A prototype (length 14 m) of the TPS vacuum system was fabricated. Two bending-magnet chambers, 4 m long, were made with computer-numerical control machining and lubricated with ethanol to protect the aluminum surface from oil contamination. Cleaning with ozonized water was applied to decrease the rate of photostimulated desorption from the chamber surfaces. Then an automatic welding system is used to implement the side welding seams of bending chambers. The design considerations, the critical factors in fabrication, and the test results of the vacuum system prototype are presented. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3305840]