Graphite nanoplatelets consisting of one or a few graphene sheets are expected to be an excellent choice for the reinforcement phase of composites. While carbon nanotubes provide reinforcement in just one direction, graphite nanoplatelets are effective in two directions. Thus graphite nanoplatelets will yield a higher degree of stiffening and strengthening in most applications where these reinforcements are expected to be randomly distributed. In the present paper, a graphite crystal of a layered structure, which is embedded in a polymer matrix, is analyzed to investigate the effect of exfoliation. When the reinforcement volume fraction is kept constant, more uniform layer separation is shown to improve the stiffening efficiency and reduce the stress concentration in the matrix. The analysis result clearly shows the beneficial effect of full exfoliation. The analysis is further extended to the case of random distribution of graphite nanoplatelets with prediction of the resulting composite moduli. The results are discussed with a view toward developing guidelines on the dispersion quality required for the maximum reinforcement efficiency.