Abstract: The Mesozoic basin of the North Yellow Sea is a residual basin.Current petroleum exploration was mainly focused on its east part.The petrology and diagenesis and its effect on porosity of the Mesozoic reservoir were studied in this paper through analyses of casting slice,scan electron microscope,X-ray diffraction,cathode luminescence,and fluid inclusions.Sandstones in this region are dominated by arkose and lithic-arkose cemented by clay minerals,carbonate and silicon.The sandstones have suffered from various kinds of diagenesis,such as compaction,cementation,alteration and dissolution,and reached the A-period of the middle diagenetic stage.A model of diagenesis was constructed in this paper with reference to the tectonic evolution of the basin.Original porosity was severely reduced under compaction and cementation in the early diagenetic stage.Porosity and permeability of reservoirs were partially improved by dissolution of feldspar and carbonate later on,but still kept at a low level.The low porosity and permeability of the sandstone reservoirs in this area were mainly caused by the losing of original porosity and the low secondary porosity.