Abstract: The stress corrosion cracking (SCC) behavior of an ultrahigh strength steel 300M in a 3. 5% NaCl solution was studied by potentiodynamic polarization and slow strain rate tests (SSRT), and the fracture surfaces at different applied potentials were observed by scanning electron microscopy (SEM). The SCC mechanism of 300M steel in the NaCl solution at open circuit potential is attributed to anodic dissolution. Cl^-ions significantly increase the SCC sensitivity of 300M steel. At the anodic applied potential of-600 mV the anodic dissolution rate and SCC susceptibility of 300M steel increase, represented by the loss of reduction-in-area at open circuit potential increasing from 52. 6% to 99. 5%. Cracks generally initiate at corrosion pits and the SCC mechanism remains anodic dissolution. At the cathodic potential of-800 mV, 300M steel exhibits a lower SCC sensitivity due to cathodic protection. Its strength and toughness values are close to those in air. The SCC is controlled by anodic dissolution and hydrogen induced cracking. When the applied potential is more negative than #950 mV, 300M steel exhibits a higher SCC sensitivity. The SCC mechanism is hydrogen induced cracking due to a synergistic action of stress and hydrogen.