Silicene, a two-dimensional hexagonal lattice of silicon, has been synthesized recently and exhibits fascinating electronic properties that resemble graphene. The substrate effect on the electronic properties of silicene is important for the practical applications of silicene. First-principles calculations were performed for silicene on two kinds of representative inert substrates, that is, hexagonal boron nitride (h-BN) monolayer and SiC(0001) surface. The silicene–substrate interaction energies range in 0.067–0.089 eV per Si atom, belonging to typical van der Waals interaction. The characteristic Dirac cone is preserved for silicene on h-BN monolayer or hydrogenated Si-terminated SiC(0001) surface. On the other hand, the silicene becomes metallic when it is placed on a hydrogenated C-terminated SiC(0001) surface. This effect was explained by the work functions for silicene and the substrates. The present results provide some guidelines for selecting proper substrates for silicene in future microelectronic devices.