This study was performed to evaluate basic characteristics of electron beam welding process for a 9% Ni steel plate. The principal welding process parameters, such as working distance, accelerating voltage, beam current and welding speed were investigated. The AB (Arata Beam) test method was also applied to characterize beam size and energy density of the electron beam welding process. The electron beam size was found to decrease with the increase of accelerating voltage and the decrease of working distance. So, in case of high voltage (150kV), spot size and energy density of electron beam were revealed to be 0.9mm and $6.5 imes10^5W/ extrm{cm}^2$ respectively. The accelerating voltage among the welding parameters was found to be the most important factor governing the penetration depth. When the accelerating voltage of electron beam was low ($leq$90kV), beam current and welding speed did not affect on the penetration depth significantly. However, in case of high voltage ($geq$120kV), the depth of penetration increased very sensitively with the increase of beam current and the decrease of welding speed.