CFD studies on a high speed centrifugal compressor with different vaned diffuser geometries are presented in this seminar, the overall objective being to improve the stable operating range and efficiency of the compressor. Bladegen 17.2 for modelling and Turbogrid 17.2 for meshing of the fluid domain are used. Initially the CFD code is validated for NASA CC3 compressor having wedge vane diffuser. The base geometry chosen for improvement has the following major design specifications: mass flow rate-0.75kg/s, pressure ratio-4.2, speed-79,000rpm, isentropic efficieny-80% and vaneless diffuser radius ratio-1.75. With the objective of improving stable operating range and efficiency of the compressor the following vaned diffuser geometries are chosen: a) airfoil vane b) multi circular arc vane c) wedge vane, and d) flat plate vane by maintaining the same chord and radius ratio of the vane. In addition the diffuser vane is replaced by partial and lean vane diffusers. Compared to the base vane configuration, the reduction in axial width or height of the vane is known as the partial vane diffuser. Vane lean is characterized as the varying flow angle of the vane in either the positive or negative direction of the diffuser vane, either at the leading edge, trailing edge or center of gravity, compared to the impeller direction of rotation. In each case pressure coefficient, loss coefficient and diffuser effectiveness are plotted to study performance improvement of the updated diffuser design with the base model. Lean vane diffuser had shown optimum performance compared to a base design with an increase in efficiency by 4.7% due to a decrease in diffusion losses. In the case of partial vane diffuser, shroud partial 30% had shown better performance with improvement in efficiency by 3% with a stable operating range.