This paper shows numerical investigation related to hydrogen-air deflagration venting. The aim of this study is to clarify the influence of concentration gradient on the pressure histories and peak pressures in a chamber. The numerical analysis target is a 27 m3 cubic chamber which has 2.6 m2 vent area on the sidewall. The vent opening pressure is set to be gauge 10 kPa. Two different conditions of the hydrogen concentration are assumed, which are uniform and gradient. In the uniform case, 15, 20, 25, 30, and 35 vol.%2concentrations are assumed. In the gradient case, the concentration linearly increases from 0 vol.%2(at the ground) to 30, 40, 50, 60, 70 vol.%2(at the ceiling). The initial total mass of hydrogen inside the chamber is the same as the uniform case. Moreover, three different ignition points are assumed: on the rear, center, and the front of the chamber relative to the vent. The deflagrations are initiated by a single ignition source. In most gradient cases, the highest peak is lower than in the uniform case, though the initial total mass of hydrogen inside the chamber is the same as in the uniform case. This is because the generated burned gas per time is smaller in the gradient case than in the uniform case. In 15 vol.%2gradient case, however, the peak pressure gets higher than in the uniform case. This is because, in 15 vol.%2gradient case, the burning velocity around the ignition point gets faster and the flame surface gets larger, which induces larger amount of burned gas per time.