Gas surface treatment is considered to be especially effective for Titanium because of its high reactivity. In this study, we investigated the gas nitriding mechanism in titanium sintered parts produced by metal powder injection molding (MIM) process. In MIM process, gas nitriding can be surface-treated subsequently after debinding and sintering process. Then, the microstructure and nitrogen content of sintered MIM parts are considered to be greatly influenced by not only nitriding conditions but also sintered conditions. In this study, the effects of sintering time on microstructure such as nitrided layer thickness and hardness was investigated. Focus was given to the effects of specimen size on nitriding process, because the size of micro metal injection molding (μ-MIM) product is so small and the specific surface of that product is so large that the mechanical and functional properties can be subject to change by nitriding.