Aluminium oxynitride (AION) thin films were deposited by Radio Frequency (RF) magnetron sputtering on n-type silicon (Si) substrate of (100) orientation using argon (Ar) and oxygen (O(2)) gases at substrate temperature of 450 degrees C. To know the change in electrical properties with gases ratio, a deposition was carried out for 140 s with Ar:O(2) ratio changed from 1:3 to 4:3. After that, electrical properties of Metal-Insulator-Semiconductor (MIS) structure with AION was analyzed. For Ar:O(2) ratios from 1:3 to 4:3, all samples showed characteristics of negative charge. In particular, when Ar:O(2) were 2:3 and 3:3, the value of flatband voltage in normal C-V curve showed above 14 V. The composition of the AION in the film was investigated using X-ray Photoelectron Spectroscopy (XPS). The flatband voltages (V(FB)) in C-V curves were found to depend on compositions. The characteristics of photon energy band gap were obtained by UV/VIS spectrum. (C) 2008 Published by Elsevier B.V.