Conventional and activated tungsten inert gas (TIG) welding were conducted under the same welding conditions in an experiment to compare the weld penetration, temperature distribution and angular distortion. The results showed that the quantities of heat input per unit welding length were almost the same, but the shapes of the weld penetration and heat-affected zones changed drastically. The difference in angular distortion between conventional and activated TIG welding was strongly dependent on the thickness of the welded plate. A distortion analysis was developed by considering convective heat transfer in the weld pool to obtain a more detailed understanding of the temperature distribution effect on the relation between welding heat input and angular distortion. The resulting distinguishable curves for conventional and activated TIG welding, which can lead to the thickness dependence of differences in angular distortion, showed the traditional relation between the heat input parameter and the angular distortion.