The hardness of CoCrFeNi has been enhanced from Hv 100 to 495 by the addition of Al to form equimolar CoCrFeNiAl high entropy alloy (HEA) and further homogenization annealing treatment at 1100℃ for 48 hr. It is finally composed of FCC and two disordered BCC (1 and 2) crystal structures, transformed form one disorder and the other order BCC structures in as-casted specimen. It is known that the corrosion resistance of CoCrFeNiAlx is deteriorated with the amount of order BCC phase and/or x increasing, and it is the worst one when x equals to 1. However through a suitable homogenization annealing treatment to avoid the formation an order BCC phase, the equimolar CoCrFeNiAl HEA has shown the similar corrosion resistance to CoCrFeNi in 0.5M H₂SO₄ and 3.5 wt.% NaCl aqueous solution. It is speculated that the order BCC phase which is more stable than the disorder one should play an important role on accelerating corrosion current, possibly acting as a cathode in galvanic corrosion. In contrast, both disorder BCC structures diminish the tendency of galvanic corrosion. However the addition of Mn to form CoCrFeNiMn equimolar HEA, only one FCC structure, has worsened in the corrosion resistance, since the Ksp of corrosion product Mn(OH)₂ is much higher than those of Al(OH)₃ and Cr(OH)₃, leading to the less protection effect on corrosion. Besides, the absorption of O₂ and/or H₂O on equimolar HEAs has been simulated by the first principle to correlated the corrosion behaviors.