Structural properties and formation energies of monovacancy (V1), divacancy (V2) and hexavacancy (V6) in silicon have been comparatively studied with density functional theory (DFT), Stillinger-Weber (SW), EDIP and Tersoff methods. The validity and shortcomings of the three classical potentials are discussed in detail. It is found that the DFT method may provide accurate description of atomic structures and energies of vacancies. As to the empirical potentials, they cannot be used to investigate quantum mechanical effects such as Jahn-Teller distortion ocurring in the DFT relaxations. Moreover, EDIP and T3 give an outward relaxation in all cases, which is contrary to the DFT and SW directions. Therefore, they are unsuitable to the structural property calculations. Based on the results calculated mainly for structure properties and formation energies, it can be concluded that SW should be the best potential to describe V1, V2 and V6.