Compression and expansion processes of cross-linked sodium polyacrylate hydrogels under mechanical pressure were investigated. A packed spherical gel bed shows irreversible deformation when the applied pressure is decreased; the expansion behavior depends on the maximum pressure applied to the gel bed, The time required to attain a certain degree of deformation is directly proportional to the square of the total solid volume of the gel bed; this relation is very similar to that observed in expression or expansion processes of ordinary solid-liquid mixtures. The driving force of the deformation is an effective osmotic pressure gradient in the gel bed, where the effective osmotic pressure of the gel is the difference between the swelling pressure of the gel and the pressure applied to the gel, The flow rate of liquid through any gel layer can be expressed by Darcy＇s equation. The deformation ceases when the swelling pressure of each gel particle is equal to the applied pressure, Thus, the deformation of a packed gel bed can be recognized as a process of equalizing the swelling pressure distribution in the bed.