TiN has been investigated as a good candidate for a diffusion barrier of Cu. Therefore, in this study, the grain boundary diffusion of Cu in TiN film was investigated by X-ray photoelectron spectroscopy(XPS). In general, TiN has a columnar grain structure. In the relatively lower temperature, less than 1/3 of the melting point, it was observed that Cu diffused into TiN mainly along the grain boundaries of TiN. The grain size of TiN was measured by atomic force microscope (AFM). In order to estimate the grain boundary diffusion constants, we used the modified surface accumulation method. The activation energy, $Q_b$ was 0.23 eV, and the diffusivity, $D_{bo}$ was $5.5 imes10^{-12{ extrm{cm}^2$/sec.