A relationship between local structure and methylene blue (MB) decomposing ability of nanoparticles (NPs) of metallic iron (Fe0) and maghemite (γ-Fe2O3) was investigated by 57Fe Mössbauer spectroscopy, X-ray diffractometry and UV-visible light absorption spectroscopy. γ-Fe2O3 NPs were successfully prepared by mixing (NH4)2Fe(SO4)2·6H2O (Mohr's salt) and (NH4)3Fe(C2O4)3·3H2O aqueous solution at 30 °C for 1 h, while those of Fe0 were obtained by the reduction of Mohr's salt with NaBH4. From the Scherrer's equation, the smallest crystallite sizes of γ-Fe2O3 NPs and Fe0 NPs were determined to be 9.7 and 1.5 nm, respectively. 57Fe Mössbauer spectrum of γ-Fe2O3 NPs consists of a relaxed sextet with isomer shift (δ) of 0.33±0.01 mm s-1, internal magnetic field (Hint) of 25.8±0.5 T, and linewidth (Γ) of 0.62±0.04 mm s-1. 57Fe Mössbauer spectrum of Fe0 NP is mainly composed of a sextet having δ, Γ, and Hint of 0.00±0.01 mm s-1 0.45±0.01 mm s-1, and 22.8±0.1 T, respectively. A bleaching test of the mixture of Fe0 and γ-Fe2O3 NPs (3:7 ratio, 100 mg) in MB aqueous solution (20 mL) for 6 h showed a remarkable decrease of MB concentration with the first-order rate constant (kMB) of 6.7 × 10-1 h-1. This value is larger than that obtained for the bleaching test using bulk Fe0 + γ-Fe2O3 (3:7) mixture (kMB = 6.5 × 10-3h-1). These results prove that MB decomposing ability is enhanced by the NPs mixture of Fe0 and γ-Fe2O3.