In this work, an inductively-coupled rf plasma reactor was utilized in the nitriding process for surface hardness improvement of aluminium-copper alloy 2011. Substrate bias at 400V was used in the pre-sputtering step to eliminate the aluminium oxide on the samples. Plasma nitriding was carried out in a N2-H2 admixture at total pressure of 1 torr. The process length was varied from 9 to 36 hours while the input rf power and substrate temperature were varied from 100 to 300 W and kept at 400 oC, respectively. A negative bias voltage up to 400 V was used in the nitriding process. Glancing incident-angle x-ray diffraction (GIXRD) results showed the hexagonal crystal structure of AlN on samples. The roughness increased slightly when the voltage increase up to 400V and was investigated by Scanning Electron Micrograph (SEM). Electron Probe Microscopy Analysis (EPMA) and Energy Dispersive X-ray Analysis (EDX) were used to detect the N atoms in specimens. Significant increases of surface hardness are observed after plasma nitriding.