In order to study the discrete characteristics of milling residual stress, an experiment on rough/fine milling of 45 steel was carried out under dry cutting or pouring lubrication. Then the discreteness of milling residual stress was studied by mathematical statistics. The experimental results show that milling residual stress has obvious discreteness,which is larger than that of turning. The dispersion of milling residual stress mainly comes from random change of cutting process and movement feature of milling tool-workpiece. The milling conditions can affect dispersion of residual stress in a certain range.The discreteness of rough milling is larger than that of fine milling, and the discreteness of pouring lubrication is smaller than that of dry cutting.