A general twostep continuous beam model was adopted to study the effects of bearing preload force on the spindle modes and milling stability of different spindle structures. The study results show that the bearing produces effects on the stability of the milling system by changing the low order modes of the spindle. The front bearing has much more remarkable effects on spindle system modes and milling stability than that of the rear one. There are existed very close relationships between the vibration of tool nose of spindle and the milling instability with effects by the same spindle mode. The spindle mode, which excited the maximum peak vibration value, produces the milling instability most likely.