The adsorption process is the first step and a crucial step in the SCR NOx removal. The adsorption characteristics of NH3, NOx on the surface of the catalyst is very important on the understanding the catalytic mechanism of low-temperature catalysts. In this paper, therefore, the adsorption of NH3, NOx and some other small molecules on the surface of the nano-gamma Fe2O3 was studied by using the first principle and density functional theory (DFT). Particularly, the adsorption processes, in which the NH3 and NOx molecules were adsorbed at different active sites of the nano-gamma Fe2O3 catalyst surface, were evaluated and analyzed. The computational results show that octahedral Fe cation of nano-gamma Fe2O3 is the most active site and many kinds of molecules can be adsorbed to the octahedral Fe cation. On the adsorption of NH3, the adsorption energy of Fe-N configuration is large and the configuration is stable. When O-defects are present on the surface, the adsorption energy of Fe-N configuration will be promoted. On the NOx adsorption process, the adsorption products are more complicated. It can be adsorbed on both the perfect surface and the O-defective surface to form many products. The H2O and N-2 can also be adsorbed on the surface of nano-gamma Fe2O3 but the adsorption energy is small. This ensures that H2O and N-2 as product can be desorbed from the catalyst surface easily.