Abstract: An uniform rectangular Intense Relativistic Electron Beam is important to KrF excimer.The main factors that affect the uniformity of e -beam include two aspects: one is the diode design and diode electrical field, the other is the self-magnetic field. In this paper the effect of self-magnetic field on the uniformity of e -beam is studied. The intensity distribution of self-magnetic field is calculated in two cases. In one case, the retangular e-beam is supposed to be infinite long along z -direction, in the other case, finite long. The trajectories of the moving electrons in self-magnetic field are also calculated. From these calculation the beam pinching sections at 6-cm distance from diode cathode in z-direction are discribed. In the calculation, the electrical field force is taken into account, and the scattering effect of e -beam of foils and Hibachi is neglected. This is useful to improving laser quality of hundred joule level KrF excimer laser.