Abstract: In order to find the way to reduce or control hydrogen production in severe accidents for Pressure Water Reactor (PWR) plants, the reaction mechanism of chromium with water was studied, rate constants were also calculated. The quantum chemistry software Gaussian03 was used to study mechanism of hydrogen production by Chromium at B3LYP/6311++G(d,p) level under quantum chemistry theory. By applying vibration frequency analysis and Intrinsic Reaction Coordinate (IRC), the possible transitional states were searched. The rate constants were also calculated under conventional transition state theory. The results show that reaction of chromium with water is a total reaction, including two elementary reactions, Cr+H₂O→CrOH+H and CrOH+H→CrO+H₂. The reaction rate constant of the second elementary reaction, CrOH+H→CrO+H₂, is lower, so the second elementary reaction is the rate determining step. Therefore, elementary reaction, CrOH+H→CrO+H₂, should be key to reduce or control hydrogen generation.