In this paper, we proposed a novel surface cleaning and nitridation technology of compound semiconductors using gas-decomposition reactions in a catalytic chemical vapor deposition (Cat-CVD) system. An NH3 gas was used for the surface modification of GaAs(100). X-ray photoelectron spectroscopy measurements revealed that, (1) oxygen related peaks vanished by a 3 min-nitridation treatment at 150 degrees C, (2) nanometer-thick GaN films were formed on the surface by 30 min-nitridation treatments, (3) nitrided GaAs had good oxidation resistances. Atomic force microscope observations revealed that these surfaces were very smooth (root mean square roughness, 0.28 nm).