Abstract: Different geometrical designs of arc skewed slot casing treatments have been investigated experimentally to research their influences on stall margin and operating efficiency.A study on the mechanism of the casing treatment was carried out on the basis of three dimension flow field measurement.The comparison of different casing treatments revealed that the best configuration of an arc skewed slot casing treatment improved the stall margin by 27%,and the peak efficiency by 1%.Compared with an axial skewed slot casing treatment,the arc skewed one not only made an improvement in stall margin,but also ended the history that the casing treatment had to pay a loss of efficiency in order to improve stall margin.