This work has been performed to determine the critical size of the GeO2 nanoparticle for lithium battery anode applications and identify its quantum confinement and its related effects on the electrochemical performance. GeO2 nanoparticles with different sizes of ∼ 2, ∼ 6, ∼ 10, and ∼ 35 nm were prepared by adjusting the reaction rate, controlling the reaction temperature and reactant concentration, and using different solvents. Among the different sizes of the GeO2 nanoparticles, the ∼ 6 nm sized GeO2 showed the best electrochemical performance. Unexpectedly smaller particles of the ∼ 2 nm sized GeO2 showed the inferior electrochemical performances compared to those of the ∼ 6 nm sized one. This was due to the low electrical conductivity of the ∼ 2 nm sized GeO2 caused by its quantum confinement effect, which is also related to the increase in the charge transfer resistance. Those characteristics of the smaller nanoparticles led to poor electrochemical performances, and their relationships were discussed.