Abstract: The anodic oxidation of annealed pure aluminium (99.99%) was carried out in 15% H_2SO_4 at different electrolytic conditions. The positron annihilation lifetime spectra of the anodized aluminium were measured and fitted into three lifetime components. The second component(τ_2, Ι_2) was assigned to the positron annihilation in the anodic film. It has been deduced that, from the Ι_2 curves, the film thirkness is propertional to the cell voltage and reaches a limit under constant cell voltage and temperature and that, form the τ_2 curves, the crystallinity of the anodic film increases with increasing cell voltage and anodizing time. The local crystallization in the anodic film would induce the void-like defects and hence result in the film less compact. An idea for improvement of mechanic properties of the anodic film on aluminium has been proposed.