Using density-functional theory (DFT) combined with non-equilibrium Green's function (NEGF), we have investigated the transport properties of carbon nanotubes with S–S, M–S, and M–M heterojunctions. The results show that the local states associated with topological defects arise at the junctions. The position and width of local states strongly depend on the configurations of the topological defects and their arrangement. The (7, 0)–(8, 0) and (8, 0)–(9, 0) heterojunctions present semiconducting characteristics. The (6, 0)–(9, 0) heterojunction maintains metallic properties. However, the 5/6/6/7 defects in the nanostructure decrease the electronic transport. More importantly, our results indicate that the I–V characteristics of the heterojunctions could be effectively controlled by gate voltage.