Cu2O nanocubes with different size (ranging from 20 nm to 400 nm) were prepared by a seed-mediated method to systematically explore the strong size-dependent properties in photocatalytic degradation of methyl orange (MO). Cu2O nanotubes were characterized by TEM, XRD, UV-Vis measurements. The size-dependent photocatalytic efficiency of the Cu2O nanocubes was evaluated by degradation of methyl orange (MO) in water under visible light (λ > 420 nm) irradiation. Furthermore, the photocurrent, linear sweep voltammetry (LSV) and electrochemical impedance spectra (EIS) measurements were applied to elucidate the size-dependent properties of Cu2O nanocubes, which demonstrated that smaller Cu2O nanocubes with certain length (30 nm) showed higher current density, faster electron transfer and lower rate of charge recombination in their exposed (100) facet. Therefore, 30 nm Cu2O nanocubes showed stronger visible light absorption capacity and higher photocatalytic activity in MO degradation among a series of nanocubes (20, 30, 100, 130, 200 and 400 nm) and their corresponding photocatalytic activities decreased with increasing the particles sizes.