A study of combustion experiments with premixed lean hydrogen-air mixtures was conducted in a vessel consisting of two compartments connected by a diameter-variable vent. Effects of various parameters (such as hydrogen concentration, vent diameter and initial pressure) on pressure loads of the combustion processes including mainly the peak pressures and the rates of pressure rise were investigated. Relations of Bow speeds of hot burnt gases through the vent and the pressure loads of combustion were adopted. The ignition mechanism in the main combustion compartment was discussed. The combustion process was classified into three patterns according to the pressure-time histories and the flow characteristics in the main combustion compartment.