The microstructures and electronic structures of nitrogen doped tetrahedral amorphous carbon (ta-C ∶N) have been simulated and investigated using molecular dynamics within the framework of density functional theory. There are no obvious relation between the coordination of nitrogen and the density of ta-C ∶N. The most common form of nitrogen bonding was found to be three coordinated. Two coordinated and a small quantity of four coordinated nitrogen atoms were also found. The addition of nitrogen caused a big increase in the sp2 fraction of carbon. The states near Fermi level increased in density as the contents of nitrogen atoms increased. Fermi level was not found to shift obviously.