In order to evaluate the combustion efficiency of Al powder in Al-H₂O reaction, the reaction of Al powder with particle size of 50 nm, 2 μm, 13 μm and 29 μm and H₂O was studied from electrochemistry. An evaluated system to characterize the Al-H₂O reaction with parameters including corrosion potential, corrosion current density and corrosion potential temperature coefficient was initially established by polarization curves of Al-H₂O reaction at different temperatures. The experimental data were fitted and analyzed to discuss the behaviours of Al-H₂O reaction affected by particle size and temperature. The electrochemical thermodynamics of Al-H₂O reaction was studied by Faraday′s law and Gibbs-Helmholtz equation. Results show that with the reduction of particle size of Al powder, the corrosion potential (E_corr) of Al-H₂O reaction decreases and the Al-H₂O reaction easily takes place. The decrease rate of E_corr is relatively significant when the particle size is less than 2μm. The combustion efficiency of Al powder with 50 nm is 90.6% in Al-H₂O system, which is much higher than that of Al powder with 2μm at 25 ℃ (66.7%).