Reduction of CO2 emission is recognized as an urgent issue in the iron and steel industry. One feasible method may be the utilization of H2 gas as a reducing agent in blast furnace (BF) ironmaking. In order to keep the BF condition stable under high H2 operation, it is necessary to understand the effect of H2 and H2O gas on the disintegration behavior of iron ore sinter in the upper part of a BF. Therefore, the reduction disintegration mechanism of iron ore sinter at 773 K under N2-CO-CO2- H2-H2O gas was evaluated. Reduction experiments were carried out under N2- 20%CO-20%CO2 (0% H2) and N2-12%CO-17.7%CO 2-8%H2-2.3%H2O (8% H2) using a 25 g sinter sample, varying the reduction time between 1.8 and 7.2 ks. Reduction degree of the sinter was calculated in terms of the mass difference between before and after the reduction. A tumbling drum test was carried out for 1.8 ks to evaluate the strength of the reduced sinter. Reduction degree of the sinter by 8% H2 gas steadily increased with time. On the other hand, it was retarded once when reduced under 0% H2. At the same reduction time, the reduction degree obtained by 0% H2 showed a lower value than that by 8% H2. At the same reduction degree, however, the RDI value of the sinter reduced by 0% H2 gas is higher than that by 8% H 2. This is because CO gas reduction proceeds mainly near the surface of the sinter particles, while H2 gas reduction tends to proceed inside the particles.