Abstract: The effect of different heat input on HAZ microstructure evolution in TIG welding was studied on 2.8 mm Inconel-718 sheets with wrought, as-cast and homogenized conditions. It was investigated that the microstructure and the grain size of all base metal and HAZ of welding joints with different heat input. With the heat input increased, precipitates dissolved in large quantities and equiaxial grains grew greatly in HAZ of the wrought joints. On the other hand, segregated structure was dispersed and a large number of residual precipitates was kept in HAZ interdendritic area of as-cast and homogenization joints, consequently, dendritic structure converted to equiaxed austenite gradually because of the widened width of intradendritic matrix and grain grew slightly because of precipitates' pinning effect on grain boundary migration. There were more precipitates in as-cast joints HAZ than homogenized sheet's joint result in more dendritic structure feature.