[en] In this paper, the effect of welding conditions on some characteristics of weld bead in electron beam welding was made clear, concerning such superalloys for the nuclear plants as Hastelloy-type, Inconel-type and Incoloy-type ones. Obtained conclusions may be summarized as follows, using technical symbols which are given meanings in this report. 1) The weld defects are R-porosity at the root of the penetration and a microcrack observed continuously in the interdendritic boundary of weld metal and the grain boundary of heat affected zone. 2) R-porosity occurs within the individual areas on W sub(R)-a sub(b) diagram. Superalloys can be evaluated in the susceptibility to the R-porosity in terms of S sub(P) and Δa sub(b). 3) From the relation between heat input and microcracking percentage, microcracks occur at and below a certain heat input. This critical heat input is herein defined as q sub(cr) to avoid microcracks which is regarded as one of the proper criteria to evaluate the superalloys in the susceptibility to microcracking. 4) Most microcracks are apt to occur when h sub(C)/h sub(N) comes near to 1.0. When microcracking position is arranged on h sub(P)-h sub(P) diagram, h sub(C)/h sub(P) generally tends to decrease with the increase in h sub(P) in all the superalloys except for Incoloy 800. Moreover, this value is scattered in the individual area of h sub(N)/h sub(P). (auth.)