A simulated annealing (SA) method based on molecular dynamics is employed to reveal atomic structures of asymmetric tilt grain boundaries (ATGBs) in MgO. sigma 5 and sigma 13 ATGBs with the [001] tilt axis are systematically investigated. The ATGBs after SA simulations dissociate into saw-toothed nanofacets composed of multiple structural units. These nanofacets are lower in GB energy than those obtained from a gamma-surface method with structural optimization, demonstrating the importance of SA-based methods for obtaining low-energy structures of ATGBs. For most of the sigma 5 ATGBs, the nanofacets consist of only structural units of sigma 5 symmetric tilt GBs (STGBs). For the sigma 13 ATGBs studied, their nanofacets do not consist of only sigma 13 STGBs but always contain non-sigma 13 structural units, which probably results from a large difference between the excess volume of sigma 13(510) and sigma 13(320) STGBs. It is also found that ATGBs have a larger number of metastable structures whose GB energies are close to the lowest energy structure than STGBs, due to the fact that ATGB nanofacets are more tolerant of variation in facet junction, structural units and their arrangement. Consequently, the lowest energy structures have low probabilities of being formed than metastable structures.