In axial flow compressors and fans, the stable working range is restricted by the so called stall line where operation may become unstable and simultaneously a short fall in compressor performance appears. Stall margin improvement has been a major task and experimental observations over the last two decades have shown that modifying the shape of the outer annulus wall, above the tips of rotor blades, is effective in delaying the onset of stall to lower mass flow rates. A wide variety of wall modifications or casing treatments have been tried with a range of stall flow improvement which can amount to about 20% .(of the stalled flow) An alternative technique for stall margin improvement in a single stage axial flow machine has been studied and results will be described in the present thesis. The technique for delaying stall involves a large scale treatment to the outer casing of the compressor which extends partly over and mainly upstream of the rotor. The operating principle appears to be that as the flow is reduced towards the stall point some radially unbalanced cells are transported from the blade tips where they are collected in the treatment , turned by the treatment vanes , and re-introduced to the main flow upstream of the rotor. A rotor with and without outlet stators has been tested ( and also with and without casing treatment) over a range of speeds and flow conditions. Also the flow condition inside the recessed casing has been simulated by means of an existing computer code known as PHOENICS.