A rotor axle of a high speed permanent magnet machine including a first part and a second part, both manufactured from a magnetically non-conducting material. The parts include an inner end and an outer end, and they can be axially interconnected one after the other by the inner ends to constitute an integral rotor axle. Their outer ends include bearing structures to fit the rotor axle with bearings. A cylindrical and solid permanent magnet is provided coaxially with the first part and the second part in between them and magnetized perpendicularly to the axial direction of the cylinder. The inner end of the first part of the rotor axle includes a cylindrical external surface with an external thread, the inner end of the second part includes a cylinder sleeve having an internal base part corresponding in shape to the cylindrical permanent magnet to constitute a space for the permanent magnet.