The scalability of electron beam pumped Kr F laser amplifier has beendiscussed as a typical example of ultra high power gas lasers. Energy depletion due to the amplified spontaneous emission is the final limitingfactor for the large scale Kr F laser amplifier. According to the scaling law of the amplified spontaneous emission, the experimental simulation of 1/1000 model has been demonstrated by using 500J Kr F laser with modularelectron beam diodes. The 500J Kr F laserat high pumping rate of 1MW/cc made it sure that the largevolume Kr F laser at low pumping rate of 100k W/cc will provide 500k J with high intrinsic efficiency above10%. Large scale Kr F laser is the most promising high power laser for fusion reactors.