<p>In this work, axial slots were numerically optimized to improve stall margin of a subsonic centrifugal compressor with a vaned diffuser. The shape of each axial slot consists of a few piecewise elliptic arcs, with the control points moved vertically and horizontally on the meridional plane to change the shape of axial slots. In the first optimization, the performance of a centrifugal compressor was calculated in 9 cases, where the control point changed the overall shape of the axial slots, and the response surface was created using the least squares method. Thereafter, the front part of the firstly optimized axial slots was further improved for enhancing the stability of the centrifugal compressor. In the second optimization, the response surface was made using the thin plate spline (TPS) interpolation based on 12 different shapes of the axial slots, because stall margin is highly sensitive to the front shape. In the first and second optimized cases, stall margin increased by 4.069 % and 4.952 % respectively, while the design efficiency decreased by 1.751 % and 1.649 % respectively. Based on the numerical results, an attempt has been made to explain why the performance changes depending on the shape of axial slots in a centrifugal compressor.</p>