A volute is the only circumferential asymmetric component in a centrifugal compressor, and thus, it should account for the circumferential asymmetry of the flow in a vane diffuser. This study performs a transient numerical analysis to investigate the effect of a volute on the flow in the vane diffuser of a centrifugal compressor under three operating conditions (near-stall, middle, and high mass flow). We compare numerical and experimental performance of the compressor, including polytropic efficiency, total pressure ratio, and unsteady pressure on a diffuser vane. The numerical scheme is proven valid owing to the fact that the numerical and experimental results considerably agree well with each other. Under middle and high mass flow conditions, the time-averaged static pressure recovery and the total pressure loss coefficients for all the diffuser passages indicate that the performance of the passages near and upstream of the volute tongue is affected negatively by the volute, whereas that of the passages downstream of the volute tongue is less affected. Under near-stall condition, the performance of all the passages is disturbed, and the diffuser passage marked as DP 3 demonstrates the worst performance. Investigation on the time-averaged aerodynamic forces, loading, and pressure on the vanes yields results that are consistent with those of the investigation on the performance of the passages. The harmonics with 0.5f b and f b , which are included in the unsteady loading and pressure on the pressure and suction sides of the vanes, are dominant, where f b is the impeller main and splitter blades passing frequency. Their amplitude values increase as mass flow deviates from the middle mass flow condition. Under middle and high mass flow conditions, the harmonic with 0.5f b is affected more negatively because of the larger amplitude on the vanes near and upstream of the volute tongue than those downstream, whereas the harmonic with f b is less affected by the volute. Under the nearstall condition, the transient vorticity fields along with the harmonics of 0.5f b and f b are investigated to evaluate the performance of the diffuser passages. DP 3, which is located at approximately 90 downstream of the volute tongue, suffers the strongest flow deterioration and is inferred to stall first. Further researches for designing more matching diffuser/volute combination will be performed by referring this study.