Publisher Summary This chapter derives the governing equations that incorporate the effective stress and Darcy's law based on a gas-solid two-phase model and explains the onset of the flooding phenomenon. Furthermore, an experiment is carried out in a rectangular container to test our hypothesis of flooding, and is compared with computation based on the linealized equations of present theory in terms of particle velocity and interstitial gas pressure. Smooth operations are required in such powder handling processes as feeding and conveying. When fine powder includes a great deal of air in such processes, a phenomenon called flooding can suddenly happen because of some external disturbance. It is a kind of fluidization of powder. If the flooding occurs, powder shows a significant fluidity and it becomes difficult to control the powder flow, resulting in various troubles. The flooding phenomenon is explained by using a gas-solid two phase flow model with a view of effective stress and Darcy s law.