Extraordinarily thick and porous oxide films are obtained on Type 304 stainless steel. The material was polarised in 5 kmol m(-3) H2SO4 solution at 50-80 degreesC with applied potential modulated as square wave. The films formed are classified into two types according to applied potentials. For the anodic type, Fe and Cr actively dissolve at the lower potential of the square wave, E-2, into Fe2+ and Cr2+, then re-oxidise at the higher potential, E-H, to be mainly Cr3+ oxide. Alternatively, for the cathodic type, Fe and Cr dissolve at E-H, which is in the transpassive region, then the dissolved Cr6+ ions are reduced into Cr3+ oxide at E-L. Transmission electron microscopy showed that the porous film consists of nanocrystalline grains of around 20-30 nm in diameter, and has many pathways linking each other to be penetrated by ions and water molecular easily. The porous layer is basically composed of Cr2FeO4 spinel oxide with excess Cr. The chemical composition, particle size, and also lattice parameter could be controlled by changing E-H, E-L, and the width of the potential pulse, t(H), and t(L). The porous oxide films formed by square wave potential pulse polarisation might be applied for various surface functional materials, i.e. a carrier of catalyst, oxide electrode, and biocompatible materials.