Abstract: To clarify the essential properties of a representative unit of cellular materials, an experimental study and computer simulations on the mechanical properties of nickel foam were performed, and the results agreed well. Considering a discrete elastic beam as representative unit and combining continuous medium mechanics theory an ideal mechanical model for porous materials was established and its macroscopic constitutive relation was given. The relation between the anisotropy nature of representative unit and material constants was discussed. The results showed that the porous material consisting of periodically arrayed units exhibited macroscopic anisotropy. Only when the units were arrayed randomly without order, the porous material exhibited statistical macroscopic isotropy. At the same time, errors in the literature were pointed out.