The effects of Al content on microstructure and properties of Monel alloy were studied by OM, SEM, an electrochemistry method and a mass loss method. The results show that the area ratio of divorcing eutectic γ′ in the interdendritic area gradually becomes larger with increasing of Al content from 2% to 4% (mass fraction). The mechanism of corrosion in the simulative sea water is pit corrosion under the conditions of different Al contents. Meanwhile, as the Al content increases, the corrosion resistance of the alloy in the simulative sea water increases first but then decreases. The Monel alloy with 3% Al addition has the best corrosion resistance in the experimental range.