Ab initio all-electron molecular-orbital calculations have been carried out to study the structure and relative stability of small silicon clusters (Si n , nϭ7-11). A number of low-energy geometric isomers are optimized at the second-order Møller-Plesset ͑MP2͒ MP2/6-31G(d) level. Harmonic vibrational analysis has been performed to assure that the optimized geometries are stable. The total energies of stable isomers are computed at the coupled-cluster single and double substitutions ͑including triple excitations͒ ͓CCSD͑T͔͒ CCSD(T)/6-31G(d) level. The calculated binding energies per atom at both the MP2/6-31G(d) and CCSD(T)/6-31G(d) levels agree with the experiments. For Si 7 , Si 8 , and Si 10 , the lowest-energy structures are the same as those predicted previously from the all-electron optimization at the Hartree-Fock ͑HF͒ HF/6-31G(d) level ͓Raghavachari and Rohlfing, J. Chem. Phys. 89, 2219 ͑1988͔͒. For Si 9 , the lowest-energy isomer is same as that predicted based on density-functional plane-wave pseudopotential method ͓Vasiliev, Ogut, and Chelikowsky, Phys. Rev. Lett. 78, 4805 ͑1997͔͒. Particular attention has been given to Si 11 because several low-energy geometric isomers were found nearly isoenergetic. On the basis of MP2/6-311G(2d)//CCSD(T)/6-311G(2d) calculation, we identified that the C 2v isomer, a tricapped trigonal prism with two additional caps on side trigonal faces, is most likely the global-minimum structure. However, another competitive geometric isomer for the global minimum is also found on basis of the MP2/6-311G(2d)//CCSD(T)/6-311G(2d) calculation. Additionally, calculations of the binding energy and the cluster polarizability offer more insights into relatively strong stability of two magic-number clusters Si 6 and Si 10 .