Abstract: Slow tensile stress corrosion tests and stress corrosion crack growth tests were used to study the stress corrosion behavior of AISI4340 steel in water at 100 ℃ containing saturated oxygen and/or 0.1 mol·L^-1 Cl^-. The results show that the presence of oxygen or Cl^- in water at 100 ℃ could increase the stress corrosion tendency of AISI4340 steel. The stress corrosion tendency in deoxygenated 100 ℃ water containing Cl^- was not significant, and the slow tensile fracture retained some ductile fracture characteristics; complete brittle fracture of AISI4340 steel occurred in the high temperature water containing saturated oxygen, and the stress corrosion tendency was significant. Oxygen or Cl^- could increase the stress corrosion crack growth rate of AISI4340 steel in water at 100 ℃. There was an interaction between oxygen and Cl^-, so their co-existence significantly increased the stress corrosion tendency and caused rapid crack growth after cracking.