In order to find a way to obtain stable α″-Fe 16X2 phase, the formability and thermal stability of α′ (bct) phase were discussed. According to a rigid sphere model, we concluded that the less formability of B for the α′ phase is due to its large atomic radius. We elucidated the difference of thermal stability of α′-Fe-X, taking into account their decomposition process. While, the decomposition of α′-Fe-N progresses only by the migration of N, without changing the bone structure of Fe lattice, the additional energy is needed to break the original α-Fe lattice in the cases of α′-Fe-B and α′-Fe-C. Therefore thermal stability of α′-Fe-B and α′-Fe-C is higher than that of α′-Fe-N.