In recent years, high speed direct drive spindle motors are obtaining more and more attentions due to their compact structure, high efficiency, low vibration and noise. Compared with those of high speed machines used for electric vehicles (EV) and refrigerator compressors, the shaft of a spindle motor is always thicker to satisfy the mechanical support requirement. When the overall outer diameter of a spindle motor is further limited for practical use, the rotor space will be restricted on both the inner and outer side, which makes the machine design process a more challenging work. This paper presents a detailed design procedure of a high speed interior PM spindle permanent magnet (PM) motor. With large volume consumption for the shaft, the rotor space is fully utilized by adopting the inverted interior V shaped PM rotor structure. Firstly, the split ratio is optimized for maximum torque density. Then, the rotor parameters, including the magnet thickness, PM pole arc, angle between adjacent PMs and the bridge width are optimized by finite element analysis (FEA). The simulated results show significant improvement on output performance, i.e. the torque density is increased by 22%. Finally, the analysis results are verified by experimental tests on a 11 Nm, 12000RPM prototype machine.