A method observing both radial and axial vibrations of time domain and frequency spectrum was studied to investigate the axial load effect of a flat-type brushless dc fan motor operating with and without the rated axial thrust force. When the developed small motor does not exhibit an intrinsically excited unbalanced magnetic force (UMF), the axial force leads to a minor contribution to the growth of the axial vibration. However, when the UMF is intrinsically excited, the preload can enormously decrease the undesired axial vibration. The ratio of the axial vibration of the prototype with the asymmetric magnetic sheet to the one without the sheet is experimentally observed around 55 percent. It demonstrates that axial load plays an effective and crucial factor in controlling the axial vibration of the mobile fan motor.