In our treatment, crystals are considered as elastic continual, so that any strained vol-um element can be simulated by an elastic dipole. The effective moment of the dipole is found and classified into a permanent part and an induced part. The former depends upon the elastic property of the volum element and the surrounding matrix; while the latter defends upon the strain state of the medium. These two parts of the effective, moment make the medium aquires paraelastic and dielastic charateristies respectively.The displacement field produced by elastic dipole in an isotropic, unbounded, elastic continuum is given. Taking relaxation into account, we obtain the expression for interaction between elastic dipole and the strain field produced by other sources. This expression is simply the Kroner's formula with by some higher order terms added to it. In treating the problems of the interaction between the dislocation and a solute atom and that between two symmetrical centers, the results agree in general with the previous studies but with some higher order correction terms included.