Packing density of perfect binary mixtures of uniformly sized large and small spheres has been developed from the structural analysis of the mixture. Analogy has been made to a homogeneous random mixture from which the packing structure of large spheres has been allowed to expand due to the trapping of small ones in their contact zones. On this basis, the packing density of the local structure of the mixture for different numbers of small spheres trapping at the contact points of large spheres and the corresponding probability of occurrence have been determined by the application of statistics and theories of stochastic processes. The resulting expression has shown to be valid for all proportions of the components but for size ratios less than 0.3. A simpler expression for the packing density of perfect binary mixtures has been developed for all values of size ratios but for proportions of large spheres less than 0.6; this equation giving results which are indistinguishable from those of the more complicated expressions. The validity of these equations has been verified by experiments.