Immersion experiment was carried out to investigate the CO 2 corrosion behavior of low-alloy steel used for pipeline at vapor-saturated CO 2 and CO 2 -saturated brine conditions by revealing microstructure, corrosion rate, corrosion phases, surface/cross-section morphology, and elements distribution. The results demonstrate that mass losses at both conditions are approximately linear to corrosion time with log scale, and the corrosion rate at vaporsaturated CO 2 condition is greatly lower than that at CO 2 -saturated brine condition. The main corrosion phases consisting of inner layer and outer layer are Cr-rich compounds and FeCO 3 crystals, respectively. Corrosion products on the coupon surface produced at vapor-saturated CO 2 condition are formed locally, while the ones produced at CO 2 -saturated brine condition deposit on the coupons surface homogeneously. That distinction is caused by the difference of formation process of corrosive solution. According to the experimental results, corrosion mechanisms of low-alloy steel at both conditions are discussed to illustrate CO 2 corrosion behavior.