Nano-sized Fe 3 O 4 particles that were covered with a porous layer of nano-sized TiO 2 (TiO 2 -masked Fe 3 O 4 ) were prepared by simply controlling pH of aqueous slurry solution containing the two particles that have different values of isoelectric point (IEP coating method). The prepared sample showed an effective masking of red-brown color of Fe 3 O 4 particles into a pale yellow. XPS and TEM measurements confirmed an effective covering of TiO 2 particles to the Fe 3 O 4 surface. XANES analyses revealed the transformation of some Fe 2+ ions into Fe 3+ in the Fe 3 O 4 structure (formation of Fe 3 O 4+δ ) due to a close interaction between Fe 3 O 4 and TiO 2 during the IEP coating, which was not observed in physical mixture of dry powders of Fe 3 O 4 and TiO 2 . Compared with pure Fe 3 O 4 or the physical mixture, TiO 2 -masked Fe 3 O 4 particles exhibited an activity enhancement for both catalytic oxidation of 1,2-dichloro benzene and thermal incineration of catalyst-embedded polyethylene and polystyrene. Temperature-programmed reduction of CO and temperature-programmed surface reaction of CO with O 2 revealed that oxygen vacancy sites on Fe 3 O 4+δ played roles for the adsorption and reaction of CO, and close TiO 2 provided oxygen to the Fe 3 O 4+δ sites. Compared with pure Fe 3 O 4 particles, TiO 2 -masked Fe 3 O 4 particles also provided a good color-masking effect when embedded in polymers and improved the thermal stability of base polymers for high temperature fabrication.