The corrosion behavior of 3%Cr carbon steel was investigated under high pressure carbon dioxide environments, containing small amount of hydrogen sulfide, to simulate the condition of high carbon dioxide containing natural gas transporting offshore pipeline. It was systematically studied under high pressure carbon dioxide (range from 80 and 120 bars) with variation in other key parameters (temperature, water partitioning and hydrogen sulfide concentration). The corrosion rates were tested using High Pressure and High Temperature (HPHT) Autoclave and measured using the techniques such as linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), iron count and weight loss (WL). The surface morphology and the composition of the corrosion product layers were analyzed by using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The results showed that in the water-rich phase, the addition of 200 ppm hydrogen sulfide instantaneously decreased the corrosion rate of 3%Cr at both 25°C and 80°C (80bar and 120 bar). It was also observed that the corrosion rates of 3%Cr carbon steel at 120bar and 80°C in the water-saturated CO2 phase are negligible. The addition of hydrogen sulfide increased the corrosivity of this phase and within the Minimum Acceptable Corrosion Rate (MACR) of 0.1 mm/year.