The electronic and optical properties of vanadium dioxide are investigated in the frameworks of density functional theory and GGA+U, in detail. It is found that, the metal-insulator transition in VO2 is induced by the on-site correlation effects, accompanied with a distinct charge-transfer. Unlike that in rutile phase, the energy gap in the monoclinic phase opens suddenly and abruptly, which is consistent with the experimental observation. The calculated indirect energy gap (0.32 eV) and the direct energy gap (0.58 eV) can be used to theoretically interpret the experimental optical transmission at 0.31 eV and the optical energy gap 0.6 eV, respectively. Consequently, both of them are confirmed by our optical calculation. Furthermore, our calculated optical absorption peaks agree with the experiment very well.