The nitriding process at a metal surface results from ammonia chemisorption and subsequent decomposition into nitrogen and hydrogen adatoms. We present a theoretical approach combined with density functional theory and tight binding extended Huckel method to systematically investigate the nitriding mechanism at Cr(100), Fe(100), and Ni(100) surfaces. The present study includes evaluation of adsorption geometries, estimation of binding energies, and comparison of nitriding properties for different metal surfaces. The calculated surface band structures are in qualitative agreement with UPS photoelectron spectroscopic measurements. Population analysis at each step of chemisorption/decomposition process yields useful insight into the electron-transfer process in the nitriding process. The theoretical results provide much information about the nitriding mechanism and suggest that Ni is a better nitriding-resisting material than Fe metal and the latter is superior to Cr. This conclusion is in agreement with experimental observations.