In this paper, based on the concept of building blocks with interactions, we study the stability of models of Si(lll) 7×7 reconstructed surface. With use of tight-binding method, we calculated formation energies of various building blocks and their interactions. With these building blocks, various large unit surfaces can be constructed and their total energies can be easy estimated, without heavy calculations. We evaluate the surface energies of Si(lll) 5×5 and 7×7 DAS models, and get the results of -0.467 eV and -0.477 eV, respectively, close to the results of Qian and Chadi. Moreover, we point out that neither the 7×7 adatom model nor the 7×7 adatom-vacancy model are stable.