Abstract: The relationship between the magnification and cavity length to the set up time of the laser mode with diffraction limited divergence in high gain KrF laser is given, and its effect on the output beam is studied. The divergence of output beam from positive branch confocal unstable resonator with M = 8, L = 1.75m and with M = 13.3, L = 1.85m is measured. Under the 30kV charge voltage, the divergences of resonator M=8 and M=13.3 are 39 and 33 times diffraction -limited divergence respectively, and as charge voltage is 26kV, they are 20 and 5 times individualy. The results of the experiment indicated that the set up time of the diffraction limited mode in the resonator is influenced by magnification, cavity length and the gain saturation effect of the laser. It is necessary that the set-up time of the diffraction limited mode must be less than the saturation time of the light in the cavity in order to get a diffraction -limited mode.