A one-dimensional optimization design system based on iSIGHT was used to optimize the matching of a diffuser with a centrifugal impeller.The system was then used to optimize the geometry and matching characteristics of a low pressure stage centrifugal compressor for a MW size gas turbine.The results show that the diffuser and impeller are not well matched in the original design.The diameter ratio of the vaneless diffuser is too large,resulting in low efficiencies throughout the entire stage. The one-dimensional optimization results are used to redefine the diffuser geometry to improve the performance.The vaneless diffuser outlet diameter is reduced and the original single stage diffuser is replaced by a tandem vane diffuser.The optimization significantly improves the performance of the entire stage.The pressure ratio is increased by about 4% and the efficiency is increased by about 2%.