A permanent magnet DC brushless motor is the most suitable for high torque electric power steering. Unlike brush motors, these motors can be driven up to high revolution speeds by using field weakening control that reduces the magnetic flux. However, vibration caused by this field weakening control occurs in an 8-pole 9-slot DC brushless motor. Therefore, the issue is that high power and low vibration must be compatible. The authors focused on this issue, and investigated the mechanism of the vibration. This paper proposes a countermeasure based on the results of this investigation, and reports test results of reduced vibration.