The titanium nitride thin films were deposited on slides and Al substrates by magnetron sputtering and the variation of N/Ti atom ratios was controlled by selected N2 mass flow rate. The effect of N2 mass flow rate on the optical properties of titanium nitride films was analyzed via spectrophotometer and scanning tunneling microscopy (STM) methods. The results of reflectivity spectrum and scanning tunneling spectrum (STS) showed that titanium nitride thin films mainly conform to free-carrier absorption mechanism, and as the N content increases, the numbers of free electrons of the films reduce consequently and the plasma frequency keeps shifting to lower energy, so that the reflectivity of films is downhill and the color of films varies regularly. STS spectra of TiN indicated that the TiN film exhibits metallic optical property and the band gap is 1.64 eV.