TiN-based contacts were fabricated to clarify the role of TiN layer in the Ohmic contact structures on n-type GaN. As-deposited TiN/Ti/Pt/Au multilayer contact exhibits Ohmic behavior, while the contact of Ti/TiN/Pt/Au is non-Ohmic. The Schottky barrier heights were estimated from X-ray photoelectron spectroscopy measurements to be 0.3 +/- 0.2 eV for TiN/n-GaN contact and 1.0 +/- 0.2 eV for Ti/n-GaN contact, respectively, as also confirmed by current-voltage measurements. The depth profiles of time-of-flight second ion mass spectrometry (TOF-SIMS) revealed that TiN layer is a good diffusion barrier, keeping the TiN/GaN interface sharp even after being annealed at 700 degrees C. Detailed interface analysis suggests the thermal stability behavior of metal/GaN contact is much related to atoms inter-diffusion and interface reaction. Our study demonstrates that low specific contact resistivity with high thermal stability can be achieved by adding a uniform TiN layer in the metal/n-GaN contact multi-structures.