Motivated by the recent observation of CP violation in the charm sector by LHCb, we study direct CP asymmetries in the standard model (SM) for the singly Cabibbo-suppressed two-body hadronic decays of charmed mesons using the topological-diagram approach. In this approach, the magnitude and the phase of topological weak annihilation amplitudes, which arise mainly from final-state rescattering, can be extracted from the data. Consequently, direct CP asymmetry a ðtreeÞ dir at tree level can be reliably estimated. In general, it lies in the range 10 À4 < a ðtreeÞ dir < 10 À3 . Short-distance QCD penguins and penguin annihilation are calculated using QCD factorization. Their effects are generally small, especially for D ! VP modes. Since weak penguin annihilation receives long-distance contributions from the colorallowed tree amplitude followed by final-state rescattering, it is expected to give the dominant contribution to the direct CP violation in the decays D 0 ! K þ K À and D 0 ! þ À , in which a ðtreeÞ dir is absent. The maximal Áa dir CP , the direct CP asymmetry difference between the above-mentioned two modes, allowed in the SM is around À0:25%, more than 2 away from the current world average of Àð0:645 AE 0:180Þ%.