The effect of time and voltage on the fabrication of self ordered nano porous aluminum oxide structure is studied. A two-step anodization process in 0.4M sulphuric acid and at low temperature was used to prepare structure with pore diameter between 20 to 29 nm. The surface morphology, including porous structure mainly, is characterized using scanning electron microscopy. Pore density 1010 to 1011 cm-2 is found. Thickness and electrical response are performed to these films. Capacitive sensor measurements for different water concentration are presented and discussed. The gas sensibility dependences with pore diameter and thickness layer are also reported.