Abstract: The technology of local heat treatment on spherical tanks is not yet supported by standards.In purpose of solving this problem, series numerical analy ses are performed on the effect of the size of heated region on the relief ratio of residual welding stress. The results show that the thermal stress induced by local heating process itself is the key factor controlling the result of local heat treatment. Satisfied stress relief ratio can impossibly be resulted from processes normally be employed in engineering because the heated region is generally too small.The stress relief ratio can be increased remarkably with the increasing of heated region, and when it is large enough, the same efficiency of heat treatment as a whole can be obtained. However, the selection of ideal size of heated region can be obviously affected by the volume and thickness of spherical tanks, and for the same expected stress relief ratio, larger heated region should be selected when dealing with larger and thicker spherical tanks.