Aluminium oxide thin films were deposited by d.c. reactive magnetron sputtering. The deposition process was controlled by target voltage while target power was adjusted to constant value by oxygen gas flow. The maximum deposition rate obtained was 150 nm/min, 60% from the value of metallic aluminium thin film. A strong hysteresis effect could be avoided by controlling the deposition by target voltage. The oxygen gas was introduced immediate to the substrates by a punched circular gas inlet system. The O/Al ratio of the film deposited varied between 1.30 and 1.72. The films deposited were transparent insulators. The structure of aluminium oxide was amorphous and columnar. The thickness of the films varied between 200 nm and 3 mu m.