The present paper numerically investigates the stall inception mechanisms in a centrifugal compressor stage composed of a splittered unshrouded impeller and a vaned diffuser. Unsteady numerical simulations have been conducted on a calculation domain comprising all the blade passages over 360 deg for the impeller and the diffuser. Three stable operating points are simulated along a speed line, and the full path to instability is investigated. The paper focusses first on the effects of the mass flow reduction on the flow topology at the inlet of both components. Then, a detailed analysis of stall inception mechanisms is proposed. It is shown that at the inlet of both components, the mass flow reduction induces boundary layer separation on the blade suction side, which results in a vortex tube having its upper end at the casing and its lower end at the blade wall. Some similarities with flows in axial compressor operating at stall condition are outlined. The stall inception process starts with the growth of the amplitude of a modal wave rotating in the vaneless space. As the flow in the compressor is subsonic, the wave propagates upstream and interacts with the impeller flow structure. This interaction leads to the drop in the impeller pressure ratio.