Phase stability and macroscopic performances of ZrC are closely related to the behavior of native point defects. In this study, structures and stabilities of native point defects in ZrC as well as diffusion of C-related defects are investigated by first-principles calculations. It is shown that the carbon vacancy (V-C) and interstitials (C(i)s) are the dominant native point defects in ZrC. Six types of C-i configurations: two C-trimers, one C-tetrahedron, and three C-dimers are identified with low defect formation energies. The V-C has a high migration energy (4.39eV) which suggests its low mobility in ZrC. The C(i)s have low diffusion energy barriers (from 0.26 to 1.29eV) which lead to their high mobility. In addition, the impact of defects (V-C, V-Zr, Zr-C, and C-Zr) on bonding strengths of neighboring Zr-C bonds is discussed. Especially near V-C, the 1NN (nearest neighboring) and 2NN Zr-C bonds are strengthened but 4NN Zr-C bonds are weakened. Interestingly, the 3NN Zr-C bonds are almost not affected by the presence of V-C. These results may be closely related to the short-range interactions and ordering of V-C in nonstoichiometric ZrC.