© 2019 Elsevier Ltd Wire Arc Additive Manufacturing (WAAM) is a promising technology for fabricating medium to large scale metallic parts with excellent productivity and flexibility. Due to the positional capability of some welding processes, WAAM is able to deposit parts with overhanging features in an arbitrary direction without additional support structures. The dimensional quality of the overhanging parts may however deteriorate due to the humping effect, which appears as a series of periodic beadlike protuberances on the weld deposits. There has been significant research on the humping phenomenon in the downhand welding, but it is doubtful whether the existing theories of humping formation can be applied in the positional deposition during WAAM process. This study has therefore provided an experimental work to investigate the formation of the humping phenomena in the positional deposition during additive manufacturing with the gas metal arc welding. Firstly, the mechanism of humping formation was analysed to explain humping occurrence for positional deposition. Then, the mechanism was validated through experiments with different welding parameters and positions. Finally, a series of guidelines are summarised to assist the path planning and process parameter selection processes in multi-directional WAAM.