Li electrodeposition on liquid Ga substrates was carried out in PC-LiClO4 at 40 °C. It was found that interfacial flow phenomena were induced by differences in interfacial tension between the liquid Ga and the liquid Ga-Li alloy formed on the substrate under potentiostatic electrolysis at −1.0 V (vs. Li+/Li), which caused the interfacial shape change of the liquid Ga electrode. We carried out in-situ observations in order to understand why such interfacial phenomena occur between the liquid electrolyte and the liquid metal cathode. Moreover, a significant difference in electrochemical response between liquid and solid Ga substrates was also noticed employing the current-time transient technique. These experimental results suggest that the process of liquid Ga-Li phase formation by the dissolution of electrodeposited Li atoms into the liquid Ga substrate dominated during the initial stage of Li electrodeposition. In contrast, the typical peaks induced by growth of solid Ga-Li alloy phase and Li metal phase were observed on solid Ga substrates. These results provide new insight into the design of electrochemical processing involving liquid electrolyte/liquid metal interface. •Li electrodeposition on both liquid and solid Ga cathodes in propylene carbonate containing LiClO4 was carried out.•Interfacial flow phenomena induced by differences in interfacial tension between Ga(l) and Ga-Li(l) alloy were observed.•The electrochemical response is related to the formation of the liquid Ga-Li alloy phase on the liquid Ga cathodes.•Future work should take into account macroscopic hydrodynamics induced by electrochemical reactions on liquid metal cathodes.