Abstract: While studying the anti-corrosion property of 13%Cr martensitic stainless steel under oil well having CO₂, a new 15%Cr martensitic stainless steel was developed and the L-80 grade seamless tubes of the steel were manufactured. The experiment shows that the 15%Cr steel with high toughness and with the same SSC resistance property as that of 13%Cr steel, can be employed in CO₂ environment containing small amount of H₂S under the temperature up to 200℃ and that it is more economic to use the steel than to use 22%Cr duplex stainless steel.