Dependence of KrF excimer laser output energy on gas partial pressures of Ne, F2 and Kr is investigated, respectively. Collision processes related to Ne are analyzed theoretically and compared with that of other kinds of buffer gas. The output energy is extremely sensitive to the partial pressure of F2. The three body collision related to Kr is dominant in the discharge process. The pulse energy thus quadratically depends on the partial pressure of Kr. Maximum output energy of 807 mJ with an overall efficiency of 1.64% is achieved for the optimum gas mixture of F2:He:Kr:Ne = 0.315:5.985:19.5:305 kpa when the charging voltage is 28.7 kV.