Uranium dioxide (UO2) is an important fuel material in the nuclear industry, and the behavior of the 5f electrons of uranium is one of the most interesting subjects about actinide elements and their compounds. Uranium dioxide is known to be a good insulator, and it is also known that the magnetic moment on uranium ions orders antiferromagnetically at temperatures below 30K.[2, 3], The structural and electronic properties of UO2 have been studied by using X-ray photoelectron spectroscopy (XPS) and Bremsstrahlung isochromat spectroscopy (BIS).[4, 5, 6] Theoretically, Kelly and Brooks have performed density functional theory (DFT)[8, 9] calculations using the linear muffin-tin orbital (LMTO) method and the local density approximation (LDA) to study the energy band structure of UO2. They found that the LDA calculations fail to describe UO2, predicting metallic behavior contrary to the observed insulating behavior. Petit et al. calculated the density of states and the cohesive properties of UO2 using the LMTO method in the atomic sphere approximation (ASA). They obtained a smaller lattice constant, a smaller cohesive energy, and a larger bulk modulus than the experimental values. They also observed that the ground state of UO2 was still metallic. The failure of the conventional DFT has been attributed to the strong correlation of 5f electrons in UO2. Recently, theoretical calculations have been made that take into account this strong correlation. For instance, Dudarev et al. obtained the correct insulating ground state of UO2 from the local spin density approximation (LSDA)+U calculations that were proposed by Anisimov et al. In addition, Kudin et al. compared the results from DFT calculations using LSDA, a generalized gradient approximation (GGA), and a hybrid DFT functional. They demonstrated that the hybrid DFT method successfully describes the electronic and the magnetic structure of UO2. Very recently, Laskowski et al. investigated the magnetic structure of UO2 using the linear augmented plane wave method with spin-orbit coupling. They compared different LDA+U schemes by calculating the uranium electric field gradients. In this work, we report on a comparative study of the conventional spin-polarized GGA (SP-GGA) and the SPGGA+U calculations for UO2. The ground state of UO2 was found to be insulating when the electronic correlation We have performed the density functional theory calculations of UO2 using the spin-polarized generalized gradient approximation (SP-GGA) and the SP-GGA+U approach. The SP-GGA+U approach correctly predicts the insulating electronic structure with antiferromagnetic ordering, but the SP-GGA calculations predict metallic behavior. The cohesive properties obtained from the SP-GGA+U calculations are in good agreement with the available experimental results and previous calculations. The spin-polarized local density of states shows that the antiferromagnetic ordering of UO2 is governed by 5f orbitals of uranium ion. Our calculations demonstrate that the strong correlation of U 5f electrons should be taken into account for a reliable description of UO2 physics.