A comprehensive investigation of transient oxidation of magnetite to hematite was theoretically conducted, with emphasis on the influences of particle size, particle temperature, and environmental conditions on the oxidation rate and burnout time of the particles, which have been suggested to use as oxygen carries (OCs) in the chemical looping combustion (CLC). Two-step solid-state reaction and transition were applied for quasi-steady gas-phase reactions with heat and mass conservation along with transient particle heating and radiative heat transfer. Within particle size ranges of practical interests in the CLC the oxidation rate of magnetite to hematite depends on particle temperature and oxidizer concentrations. The results show that, while the temperature of magnetite particles increases continuously during oxidation, the final particle temperature at the completion of oxidation rapidly decreases as particle size increases and then slightly increases or is nearly constant. The lowest final particle temperature is for the particles around 100 μm in radius. Whereas the burnout time of the particles increases with increasing particle size, the initial particle temperature does slightly influence the burnout time with particles having lower initial temperatures needing longer time for complete oxidation.