Effects of discharge instability and gas kinetics on the laser output energy of a transversely pumped KrF laser are discussed. Time evolution of laser power at 248nm, Kr₂F^* emission intensity at about 420nm, light intensity over 580 nm representing an existence of glow and arc discharges are experimentally obtained, where fractional densities of krypton and fluorine are varied from 1 to 8% and from 0.1 to 0.7%, respectively. Krypton gas dependency of laser output energy can be explained by the gas kinetics which is based on the behaviours of excited krypton Kr^* and trimer excimer Kr₂F^* When the fractional krypton density G_Kr is increased from 1 to 4%, the laser output energy increases owing to the increase of Kr^* density. For over 4% of G_Kr, Kr₂F^* density produced by three-body quenching reaction of KrF^* excimer becomes larger. As a result, the laser output energy decreases. For more than 0.4% of fractional fluorine density G_F2, it is seen that light intensity transmitted over 580nm becomes strong remarkably in the first half cycle of the current. This remarkable change of the light intensity shows an occurrence of the discharge instability bringing termination of the lasing. Accordingly, when G_F2 is varied, the discharge instability is clarified to be a main factor of affecting the laser output energy.