For description of elastic modulus of nanocomposites polyurethane/graphene the modified mixtures rule was proposed, which takes into consideration two factors. First, this rule assumes, that in polymer nanocomposites interfacial regions are the same reinforcing element of their structure, as actually nanofiller. Secondly, real, but not nominal, characteristics values of nanocomposite components were used. This allows the quantitative description of elastic modulus of the considered nanocomposites exactly enough. Reaching of percolation threshold of graphene platelets results to the essential enhancement of elastic modulus for both structure components and nanocomposite as a whole.