The paper proposes a new method to determine the tooth cogging force in a brushless DC (BLDC) motor which is used in a hard-disk application. This force might be a source of tooth vibrations and motor noise. In the new method, the permanent-magnet (PM) poles are substituted with their equivalent currents. Then, the conformal mapping (CM) method is applied to determine the PM magnetic field. Finally, the cogging force is determined by integrating the magnetic field pressure over the tooth surface. The paper also analyzes the harmonic content of both radial and tangential force components.