The thermoelectric properties of cubic zinc-blend silicon carbide nanowires (SiCNWs) with nitrogen impurities and vacancies along [111] direction are theoretically studied by means of atomistic simulations. It is found that the thermoelectric figure of merit $ZT$ of SiCNWs can be significantly enhanced by doping N impurities together with making Si vacancies. Aiming at obtaining a large $ZT$, we study possible energetically stable configurations, and disclose that, when N dopants are located at the center, a small number of Si vacancies at corners are most favored for $n$-type nanowires, while a large number of Si vacancies spreading into the flat edge sites are most favored for $p$-type nanowires. For the SiCNW with a diameter of 1.1 nm and a length of 4.6 nm, the $ZT$ value for the $n$-type is shown capable of reaching 1.78 at 900 K. The conditions to get higher $ZT$ values for longer SiCNWs are also addressed.