Assuming the metals to be an isotropic, unbounded and homogenous elastic continuum, the tetrahedral and octahedral position energy of interstials (C,N,O and H) in b.c.c. metals (Fe, W, Mo, Ta and Nb) are evaluated by means of the concept of elastic dipole. The results show that all of them occupy the octahetral positions except H, which in W, Mo, Ta and Nb favours the tetrahedral position, and the critical radius of interstials at which the favoured position is exchanged have been given also. The activation energies of interstials moving between octahedral positions are calculated. The results are compared favourably with previous experiments.