Abstract: Based on experimental data of mechanical and physical properties, hydrocarbon concentration, as well as field test of pore-water pressure and its disappearing time, a comprehensive investigation has been conducted to study the cap beds and its sealing mechanism in shallow biogenic gas pools of the Late Quaternary in the Southeast China coastal plain. The result of this study has indicated that the gas-generating beds above the reservoirs could he considered as the cap beds. The gas cap beds in this area are primarily grey mud deposited in inner sublittoral environment and grey-greyish black mud deposited in fluvial flood plain environment. The grey mud usually occurs above the incised valleys formed in the last glaciation (such as Qiantangjiang incised valleys) and could be considered as the regional cap beds, while the grey-greyish black mud occurs only within the incised valleys and forms a local cap beds. Such parameters reflecting physical properties of sediments as the porosity, pore ratio, natural water 0ontent and pore-water pressure are related to specific surface area of particles in the sediments. The smaller the grain size is, the larger the parameter values are. Therefore, the parameter values increase successively in fine silt, silt, siltic clay and clay. As for the petro-mechanical properties , the clay has not only the largest compressibility coefficient, plastic and liquidity indexes, as well as the shear strength, but aim the weakest deformation-resistance and volumetric deformation. Consequently, the clay is the best sealing layer comparing with the silt mud and silty day. The sealing ability of the cap beds increases with its buried depth. Although the sealing mechanisms of the cap beds include physical and hydrocarbon concentration sealings, the pore-water pressure sealing probably is the most important mechanism in the shallow biogenic gas field.