Porous anodic aluminum oxide (AAO) films were fabricated electrochemically in different preparation conditions such as in the mixed electrolytes with various volume ratios of 0.3M C2H2O4 and 0.3M H2SO4,in pure oxalic acid solution under different anodizing voltage and time.The transmission spectra with the interference fringes were measured and the modified Swanepoei method was used to determine the optical constants of the free standing AAO films.The calculated thickness agrees well with the measured thickness from the FE-SEM images of the cross section,which indicates that the modified Swanepoel method is very fit for the determination of the optical constants of the free standing AAO films.Meantime,the influence of the preparation conditions on the measured optical constants and the physical thickness of the AAO films were discussed.The optical band gap is appropriately fitted to the direct transition model proposed by Tsuc in the strong-absorption region of investigated films,and is derived from Taucs extrapolation.The reasons were investigated.Acknowledgements The Authors are grateful to the support of the National Natural Science Foundation of China (Grant No.10774121),the Natural Science Foundation of Gansu Province of China (Grant No.0803RJZA102),the Foundation of National Key Laboratory of Surface Engineering (Grant No.9140C540202090C54),.the Foundation of Northwest Normal University (Grant No.NWNU-KJCXGC-03-22),and the Foundation of Education Bureau of Gansu Province of China (Grant No.0801-09).