Regarding particulate number (PN) in the exhaust gas emission of gasoline direct injection (GDI) engine vehicle, awareness of environmental protections had increased. In order to decrease the emission of particulate matter (PM), it was effective to suppress the emission by the improvement of engine combustion, and/or filter the PM by the gasoline particulate filter (GPF). In this paper, the improvement of the coated GPF was conducted to reduce the pressure drop while securing the three-way performance and the PN filtration efficiency. It was necessary to load a certain amount of washcoat on GPF to add the three-way function, but it lead to increase the pressure drop that affected the engine power. The pressure drop was influenced by the gas permeation properties of filter wall. To understand the influence factor of the wall permeability, investigations of the pore information of filter wall were carried out by utilizing the mercury porosimetry and the three-dimensional structure analysis of the pore structure by micro X-ray CT. As a result, it was confirmed that the number of percolation path was decreased significantly by the washcoat loading. In addition, the contribution of the diameter of each percolation path was analyzed by applying the permeability model. It was clarified that the amount of the narrow percolation path had influence on permeability. The washcoat method that controlled the washcoat distribution was applied to increase the number of percolation path, and the coated GPF with low pressure drop was developed.