High-speed bearings and electrical motors are gradually getting available. This is of particular interest for the application of thermal turbomachinery. The availability of high speed bearing and motor components opens the way to new applications for low power, compact, oil-free and high speed radial compressors. This paper describes the basic design of a high speed single stage radial compressor for high pressure ratios and a wide flow range using a vaneless diffuser. The target application is aimed at domestic heat pumps. A commercially available 3D viscous code has been applied to calculate the flow field through impeller and diffuser. The choice of the bearings allows using very small tip clearances. Particular attention has been paid to minimize the secondary flows and therefore reduce exit flow non- uniformities in order to improve the global efficiency. To complete the design a commercial 3D Finite Element code has been used for stress prediction as well as for the calculation of the critical blade frequencies and their corresponding modes. During the design process the impeller blade geometry has continuously been checked for machinability in order to ensure a feasible impeller using conventional 5 axis milling machines.