The normal viscous force between two arbitrary rigid spheres with an interstitial power law fluid was studied in order to establish model for wet granular materials using the Discrete Element Method. As a result, analytical integration form expressions of pressure distribution and the viscous force for two arbitrary spheres were obtained, which behaves smooth, continuous and could be reduced to the Newtonian case. Numerical comparison on the viscous force with Rodin's asymptotic solution shows a general coincidence with each other when the power index exceeds 1/3.