AbstractLow anodic efficiency of metallic Mg, which results in low discharge capacity and specific energy density, hinders the wider application of aqueous primary Mg batteries. In this work, we clarify the decisive factors effecting the utilization efficiency of Mg anodes. Anodic efficiency of several Mg anodes at different current densities is measured in 3.5 wt% NaCl electrolyte. The contribution of self-corrosion at open circuit potential and at constant applied current to efficiency of Mg anodes is compared. Additionally, efficiency loss caused by detachment of undissolved metallic portions, namely “chunk effect”, is determined with a proposed new approach. The effect of self-corrosion and “chunk effect” on anodic efficiency is assessed accordingly. The results indicate that “chunk effect” can also result in large efficiency loss of Mg anodes, especially at low current densities, which, in some cases, could exceed the loss caused by other mechanisms of self-corrosion of anode substrate. Hence, attention should be paid to both aspects when developing novel Mg anodes with high anodic efficiency, particularly for application in long-term low-power battery system.