Abstract: The hub treatment with a circumferential groove was tested on the stator hub and compared with the casing treatment over the rotor tip for a single stage axial flow compressor.The 3D flowfield at the downflow of stator and rotor blade rows,in particular its endwall flowfield,in optimum operating and near stall states were measured by a micro-five-hole probe.It was shown that stall margin can be improved not only for the single-stage(=2 96%),but also for the rotor(=2 79%),even if the onset of stall is at the rotor tip first.The reason is that the stall affected by treatment groove to retard the appearance of stall.Some initial researches have been made for improvement of performance of the single stage and rotor,as well as the relation between the rotor and stator.It is found that the trend in variation of the most performance parameters of the rotor with hub treatment is similar to that of isolated rotor with casing treatment.According to the analysis of downflow,a new flow phenomanon is clarified:the dangenous stall locates at the blade hub and near the pressure surface.The mechanism to extend stability for a hub treatment with circumferential groove is also considered.