Abstract: A method for optimizing the meridional passage of the rotor in acentrifugal compressor has been developed according to the theory of S1 and S2 stream filaments. Two optimization principles are introduced. First, the gradient of static pressure in stream line directon should be minimized as it is mainly responsible for the boundary layer separation at the hub and shroud. Second, the gradient of static pressure from hub to shroud should be lessened as possible for it is generally considered as the major reason for triggering the second flow in the passage. According to these two principles, a reasonable criterion has been set up. In order to optimize meridional passage automatically through computer programming, two practical approaches have been provided . In one approach, the passage is modified again and again by using formulas derived from the continuity equation and thermodynamic relations. In the other, a series of quadratic curves are introduced to form a series of meridional passages. In both these approaches, it is neccessary to predict the flow field by using "S₂ inverse problem" computer code many times. Then, according to the above-mentioned criterion, the optimized meridional passage can be distinguished. Two numerical examples are given in this paper. Their results are obtained from the two approaches respectively. The comparison of the static pressure distributions at hub and shroud between the original and optimized passages indicates that this method obtains access to the expected results.