Abstract: A new type of frontally tapered squared trench casing treatment was designed and tested over the rotor tip in a single axial flow compressor.Variations in the rotor tip penetration into the trench and the distance of the trench front from the blade leading edge were investigated.Through the measurements of velocity and pressure,the effects of each geometric parameter upon compressor efficiency and stall margin were evaluated.An optimal configuration has been found which gives not only 0 4% improvement in efficiency but 2% in stall margin in comparison with the smooth wall baseline.The mechanism of enhancement in the efficiency and the reason for existence of optimum configuration for frontally tapered squared trench casing treatment are considered in this paper.