Suppression methods for rotating stalls in low-specific-speed centrifugal compressors with vaneless diffusers are experimentally studied. Using a low-solidity vaned diffuser, the aerodynamic performance is improved and the rotating stall is suppressed. The main parameter which affects the suppression is the ratio of the diffuser vane leading edge to the impeller tip radius. The suppression is large where the leading edge radius ratio is less than the radius ratio at which flow begins to reverse as calculated by axisymmetric boundary layer flow analysis. For rotating stalls in vaned diffusers, the stall cell tends to be equal to 1.