In this study, the effects of annealing in a nitrogen atmosphere on the crystal structure and resistivity of tantalum nitride films was investigated. The films were deposited on silicon substrates by a DC magnetron sputtering technique, which is implemented with an optical emission spectrometer (OES) in the sputtering system to control the ratio of tantalum to nitrogen particles in the plasma on line. The results reveal that annealing in nitrogen atmosphere improves the crystal structure and raises the electrical resistivity. The thermal annealing resulted in grain growth especially when OES readings were set at 100 and 70%. This may be caused by the fact that nanograms provide the driving force to induce the grain growth process. Grain growth phenomenon is not as obvious when the OES value is decreased because other phases such as Ta2N and Ta5N6 are termed and inhibit grain boundary movement. Annealed samples have higher electrical resistivity compared to the as deposited ones, It may be caused by the escape of excessive nitrogen atoms, which act as donors to the electronic conductance, during annealing. Decreasing the OES value from 100 to 45%, the crystal phase of as deposited tantalum nitride film changes froth beta -Ta, to Ta2N, then to TaN and finally to Ta5N6.