Partial upgrading of oilsands bitumen has been considered as an effective approach to reducing the usage of diluent during the pipeline transportation of raw bitumen to its market with less economic pressure and greenhouse gases emission. Thermal cracking is a mature technology in oil and gas industry to break large molecules to smaller ones, which can be easily adopted in partial upgrading to produce a less viscous bitumen. Previous studies indicated that the temperature for thermal cracking of bitumen should be limited to 400 °C to maintain a satisfactory liquid yield. However, the corrosion of partial upgrader constructional materials under thermal cracking conditions remains a safety concern for long-term operations. In this work, a low-alloyed steel, UNS K91560, was exposed to thermal cracking of a Canadian oilsands bitumen in the temperature range of 360 – 400 °C. The samples exposed to the liquid phase experienced more severe corrosion than those exposed to the gas phase. Much higher corrosion rates were observed at enhanced upgrading temperatures. Characterization techniques such as XRD, SEM/EDS were employed to examine the formed corrosion products. The corrosion mechanisms were explored based on the characterization results.