The lowest order and origin of radial electromagnetic force are analyzed at first in machines with different slot-pole combinations and winding layers. Then the amplitude of lowest order component of radial electromagnetic force is compared in machines with same slots but different poles. Structrual finite element method is used to calculate the vibration of external rotor and the law of the lowest order of radial electromagnetic force is summarized. The main findings include: 1) Machines with slot-pole combination which produce lower order radial electromagnetic force exhibit larger vibration; 2) Vibration in machines with same slot number but more poles is larger than machines with less poles.