Abstract: A theory in the framework of continuum elasticity has been developed to calculate the totalcontribution of "atomic size effect" or "strain energy effect" to free energy of binary solidsolutions. It is found that elastic free energy consists of two parts f elastic self energy (ESE),and elastic interaction energy(EIE). The former is a function of composition alone, the latter isalso a function of atomic configuration. Minimization of total elastic free energy with respect toatomic arrangement resuIts in an ordered arrangement of atoms, which is caIIed elastic interactionordering (EIO), as it originates from elastic interaction among atoms. EIO is a kind of localordering within a "characteristic range", and it is found to be important in determining the Stateof solid solutions and structures of superlattices. The present theory also gives good explanationto the coexistence of ordering and decomposition which can not be understood in conventionaltheories.