The 16 polarized cross sections for electron-positron scattering are calculated. They can be grouped into three classes according to the behavior of the angular distribution. The three classes have singularities sin-4( θ /2) ，sin-2 ( θ /2 ) and 1 in the forward direction， respectively. The 16 polarized cross sections for electron-electron scattering are also calculated. They can be grouped into three classes according to their singular behavior in either the forward direction or the backward direction. The singularities are similar to those for electron-positron scattering. A simple physical explanation is presented for the singular behavior.