We report ab initio calculations of magnetic and spin-polarized quantum transport properties of pure and nitrogen-doped carbon atomic wires. For finite-sized wires with even number of carbon atoms, total magnetic moment of $2{\ensuremath{\mu}}_{B}$ is found. On the other hand, wires with odd number atoms have no net magnetic moment. Doped with one or two nitrogen atom(s), the carbon atomic wires exhibit a spin-density-wave-like state. The magnetic properties can be rationalized through bonding patterns and unpaired states. When the wire is sandwiched between Au electrodes to form a transport junction, perfect spin filtering effect can be induced by slightly straining the wire.