X80 pipeline steel is a low carbon, micro—alloyed high—grade steel and a fairly new steel used as pipeline material in worldwide. The material has the huge potential to be used widely for building the oil/gas transmission pipelines in the 21st century because of its high intensity and high toughness. X80 steel has been adopted on the second west—east gas transmission pipeline project in China. Whereas, there is a issue, stress corrosion cracking (SCC) is more likely to occur on X80 pipeline steel, because of its high strength and fine microstructure, it will be a vital threat to safe operation of buried oil/gas pipelines. However, the related research about SCC behavior of X80 pipeline steel in high pH carbonate/bicarbonate solution is rarely reported at present. Comparing with X65 pipeline steel, X80 steel has higher strength and finer microstructure, because of these differences, it may have some certain influence on the SCC behavior of X80 steel, and even change the mechanism of high pH SCC. Consequently, it is necessary to study the SCC behavior and mechanism of X80 steel in high pH solution. In this work, the SCC behavior and mechanism of X65 and X80 pipeline steels in high pH concentrated carbonate/bicarbonate solution are investigated by slow strain rate testing (SSRT), electrochemical test and surface analysis technique. The results show that the cracking mode of X65 pipeline steel in carbonate/bicarbonate solution is intergranular SCC (IGSCC). While the mixed cracking mode of X80 pipeline steel in high pH solution is that the crack is intergranular in the early stage of the crack propagation, and transgranular SCC (TGSCC) in the later stage, which is mainly transgranular. The cracking mode of X80 steel is associated with the microstructure and high strength of the steel. The key reason for TGSCC occurring of X80 steel is that the decrease of pH value of the crack tip during the crack propagation process. The SCC mechanism of X65 steel in high pH carbonate/bicarbonate solution is anodic dissolution (AD) mechanism. While the SCC mechanism of X80 steel in high pH solution is mixed controlled by both AD and hydrogen embrittlement (HE) mechanisms, and the HE mechanism may play a significant role in the deep crack propagation at the later stage. The high strength X80 steel consisted of fine acicular ferrite and granular bainite has a higher susceptibility to SCC in high pH solution, comparing with low strength X65 steel composed of ferrite and pearlite.