A brushless DC motor structure with a constant ratio of multiple rotor poles to slots of the stator is disclosed, which is characterized primarily by forming the stator of the motor by multiple ferromagnetic silicon steel sheets, where the ferromagnetic silicon steel sheets are provided with the multiple slots whose number is a multiple of 15, and the stator of the motor is formed by windings of the three phases, X, Y, and Z. Each phase includes 2 to 4 phase portions and each group has 5 slots. The rotor of the motor is made up of a plurality of arced magnets which are fixed orderly and equally along a ferromagnetic steel ring, and the radial direction of each arced magnet is opposite to that of the adjacent magnetic poles. An arced magnet represents a magnetic pole, and the number of the magnetic poles is a multiple of 14 or 16. By means of the aforementioned setting, the reduction of the cogging torque of the motor is achievable.