Essential geometric and operating factors controlling steady performance characteristics and starting transients of a straight axi-symmetric supersonic exhaust diffuser with zero-secondary flows are studied experimentally and numerically. Static pressure distributions along the diffuser wall and vacuum chamber pressure records are measured by a small-scale simulator using cold nitrogen gas as a working fluid. Preconditioned Favre-averaged Navier-Stokes equations with turbulence compressibility effects incorporated into low Reynolds number k-E turbulence model are employed to solve steady and unsteady flow-fields in the diffuser. The fact that the mode transition of a small L/D supersonic diffuser occurs at higher nozzle inlet total pressure than the optimum starting pressure of a large L/D supersonic diffuser is visualized numerically. In the mode transition regime, a sudden drop of vacuum chamber pressure during a starting transient observed in the measured data is numerically reproduced and the retardation of the supersonic jet impingement is shown to be the primary cause for the jumping mode transition. Abrupt shrinking of the internal shock structure is observed experimentally as the nozzle inlet total pressure decreases; however, this makes no significant alteration to the vacuum chamber pressure, as long as the nozzle inlet total pressure is greater than the diffuser starting pressure. In addition, the vacuum chamber pressure is promptly equilibrated by the low initial cell pressure, due to the rapid expansion of the under-expanded supersonic jet from the nozzle exit immediately followed by opening the diffuser exit port cover. In this case, due to the strongly reversed nozzle exhaust flow into the vacuum chamber, locally supersonic flow structure emerges momentarily in the vacuum chamber.