en Research Paper Research Paper The conical nanostructure improves the applications of alumina membranes and provides three dimensional nanometer scale systems to study the chemical and physical properties. In this study, the nano cone structure is produced in porous anodic alumina (PAA) by two-step anodizing. This conical nanostructure will improve the application of PAA membranes. This approach is novel generation of the so-called "gradually decreased voltage" technique, in which the voltage- time curve is divided into three stages and the effect of each step is investigated for different electrolytes. The effect of the decreasing voltage is examined in two types of electrolytes oxalic and phosphoric acid with a constant decrease in voltage rate. The results of SEM, FE-SEM images show the slope of the inner cone layer in the oxalic acid to be considerably larger compared with phosphoric acid. Porous anodic alumina, anodizing, phosphoric acid, oxalic acid, nanostructures, cone porous 71 78 http://ijmse.iust.ac.ir/browse.php?a_code=A-10-127-53&slc_lang=en&sid=1 Sh. Damghanian 180031947532846002532 180031947532846002532 No Department of Mining and Metallurgical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran. H. Omidvar omidvar@aut.ac.ir 180031947532846002533 180031947532846002533 Yes Department of Mining and Metallurgical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran. S. H. Tabaian 180031947532846002534 180031947532846002534 No Department of Mining and Metallurgical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran. A. R. Azadmehr 180031947532846002535 180031947532846002535 No Department of Mining and Metallurgical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran.