A clear insight into the full penetration humping (FP-humping) formation in high-speed gas tungsten arc welding (GTAW) is difficult owning to the complex and multi-coupled transport phenomena in molten pool. In this paper, sensitivity analysis and dimensional analysis are combined to study the molten pool behavior and FP-humping formation. Firstly, sensitivity analysis and an experimentally-verified numerical model are used to clarify the effect of driving forces on characteristic molten pool behaviors and defect formation quantitatively. The results show that both arc pressure and arc shear stress have considerable effects on promoting defect formation, and their significances are in the same order. The surface tension shows predominant role to suppress defect formation. Subsequently, dimensional analysis based on Buckingham it-theorem is performed to derive some physically meaningful dimensionless variables. The dimensionless humping frequency is a linear function of a dimensionless group containing characteristic molten pool variables and material properties. The reasonability of dimensional analysis result is tested by additional numerical data, and there is a good agreement. This study clarifies the physical origin of FP-humping defect in GTAW, and may also provide some fundamental guidelines for its suppression. (C) 2018 Elsevier Ltd. All rights reserved.