Abstract: The effect of frequency from 10 to 100Hz on A. C. etching of aluminum foil in hydrochloric acid has been investigated. It was found that there exists an optimum frequency f_m corresponding to the relatively highest surface area magnification under certain etch condition. f_m was also temperature dependent, it moves toward higher value with increase of temperature.The preferable frequency and temperature were 20Hz and 35℃ respectively. Based on the breakdown potential obtained from the oscillogram, the weight and the morphology of etch film, it may be concluded that with the frequency below f_m the etch film formed was thin and it was easy to be broken through to produce a higher density of pits, which would be merged with each other ultimately, leading to a loose morphology; with the frequency higher than f_m the etch film was too thick to be broken through, the etch morphology exhibited only a few large pits. Only at f_m the thickness of the etch film was suitable to produce a moderate density of pits, and at last a deep layer of roughened metal was formed, giving the maximum surface area magnification.The importance of the initial aluminum surface condition in A.C. etching was also discussed.