The electron field emission properties of amorphous carbon (a-C) films deposited using a rf magnetron sputtering system have been improved by introducing a simple method of argon and nitrogen plasma treatments at room temperature. Surface morphologies of the a-C films were investigated by scanning electron microscopy, and the nitrogen contents were measured by x-ray photoelectron spectroscopy (XPS). XPS study revealed that nitrogen was incorporated into a-C films of the samples treated with nitrogen plasma. A maximum nitrogen content of 7.7 at. % was achieved with a 45 min plasma treatment. The emission properties improved with the plasma treatment in both argon and nitrogen plasma.