9%Ni steel similar composition weld wires are applied for constructions of liquefied natural gas storage tanks by gas tungsten arc welding and the weld metals show high toughness and strength level. In this study, the microstructures of 9%Ni steel similar composition weld metals have been investigated. It was found that fine microstructure was formed at the reheated zone and contributed to the high toughness level. The nucleation rate of blocks and packets from prior-austenite grain boundaries was larger at the reheated zone than the as-welded zone and resulted in the fine microstructure of the reheated zone. Our results showed that the lowering of Ni concentration at and near the prior austenite grain boundaries by the reverse transformation caused the larger nucleation rate at the reheated zone.