In this study, the effect of welding process and heat input conditions on the angular distortion induced by bead-on-plate welding was investigated through a numerical approach. Numerical models of gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW), which were developed in a previous study, were utilized for accurate distortion analyses. The calculated relation between welding conditions and angular distortion was quantified by using the conventional heat input parameter derived from welding thermal conduction theory. The results clarified the limits of applying the heat input parameter to quantify angular distortion under the various welding process and heat input conditions. In addition, the effect of the weld reinforcement generated in GMAW on angular distortion was coordinately examined by using a parameter, defined as the ratio of the area of weld reinforcement to the square of plate thickness. The effect was generally negligible except for in the case of a thin plate. Then, a parameter of the mechanical melting region on the plate thickness section was applied to quantify the angular distortion induced by GMAW and GTAW. As the results, a unified evaluation of the effect of welding process and heat input conditions on angular distortion was successfully achieved. Thus, it can be concluded that the developed parameter of the mechanical melting region on the plate thickness is the dominant factor for accurately quantifying angular distortion.