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The dependence of ultimate tensile strength (UTS) on the parameters of low voltage and high voltage electron beam welding (LV-and HVEBW) has been analyed by changing one factor at a time (COFAAT) and by Taguchi's method for AZ series magnesium alloys. COFAAT is a conventional method for trend analysis of each parameter. However, its cost is high and a long time is needed, and interactions between parameters are ignored. On the other hand, Taguchi's method cannot only reduce the number of experiments, but also provide precise optimum parameters and predicted UTS even when a few data points are lost. According to the results of analysis of variance, the strength of a weldment is affected by oscillation of the beam, the focal position, the welding speed, the accelerating voltage, the beam current and stress relief, in order of impact. The general trend is for the optimum parameters to include a non-oscillating beam, focus at the bottom and no stress relief. With regard to welding speed, accelerating voltage and beam current, there are no specific trends. Finally, the differences in welds between LV-and HVEBW for AZ series forged alloys have been examined to study the mechanism of pore formation, which results from the dissolution of precipitates to reform molecular oxygen. It is also emphasised that the area and shape of the weld cross-section are related to the input power and welding speed.