ABSTRACT The electron-velocity distribution function of the discharge plasma in a rare gas halide excimer laser was diagnosedby the Thomson scattering method. The measured electron-velocity distribution function shows that the predictedvalue by the calculation is accurate. Those results assure that the calculation method which is based on the kineticcalculation using the circuit equations, the Boltzmann equation and the rate equations is effective in the excimerlaser simulation. 1 INTRODUCTION The electron-velocity (electron-energy) distribution function is a key parameter of the discharge plasmas in rare gashalide excimer lasers because the excimer formation rate is greatly influenced by the electron-velocity distributionfunction. For example, the laser output energy of the Ne-based gas mixture is higher than that of He-based gasmixture because of the difference of the electron-velocity distribution . The electron-velocity distributionfunction is the most essential parameter in the excimer laser simulation. The electron-velocity distribution functionhas been calculated by the kinetic simulation which is based on circuit equations, the Boltzmann equation and therate equations 1,2 However these former works are based on uncertain experimental parameters to calculate thefunction. Moreover, these calculated electron-velocity distribution function have not been proved experimentally.