A simple model about transition impurities in semiconductors is proposed, and the impurity energy levels and wave functions produced by substitutional and interstitial atoms in silicon are calculated by Green's function method. It is shown that the properties of these two kinds of impurities are remarkably different. The substitutional atom can produce impurity energy level only when Vd, the atomic level of d state, is below the top of valence band. It's wave function is mainly dangling bond state, and gradually turns into bond state when the energy level approaehs the edge of conduction band. The interstitial atom can produce impurity energy level only when Vd is above the top of valence band. It's wave function is mainly d state of central atom and gradually turns into weak antibond state. Finally, the chemical trend and some experimental facts of transition impurity energy levels are qualitatively explained.