The influence of fluid flow on the slope stability of packed beds has been investigated. By taking the interparticle cohesive force into consideration, the behavior of a packed bed of fine powdered material percolated by a fluid has been studied. The volume forces induced by fluid percolation, gravitation and the interparticle cohesion act on the packed bed which is considered as a continuum. The force balance analysis leads to a relation between the pressure gradient, interparticle cohesion and the maximum slope steepness prior to the failure of the bed surface. The resulting equation is found to be valid for beds of fine powdered material conveyed in pneumatic transportation.