Most concerns have focused on the arsenic (As) contamination in wastewater. Montmorillonite (MMT) has been proved to be a good adsorbent for removal heavy metals existing as cation in ground water while it is invalid for anions. However, arsenic usually exists as anions in aqueous. Accordingly, suitable modifications on MMT need to be done before using. This paper presents the results that a kind of commercial MMT has been modified by iron oxides under normal and inverse titration conditions. The x-ray diffraction (XRD), Thermal Gravimetry Analysis and Differential Thermal Analysis (TGA-DTA) and SBET analysis have been employed to elucidate the modification mechanisms. Modification characteristic analysis illustrated that the iron oxide associated with MMT by coated style as polymerization of hydroxyl-Fe (Fe (OH) 3) under inverse titration condition. Under normal titration condition, on the other hand, the iron oxide associated with MMT predominant by intercalation as goethite (FeOOH). The arsenic removal efficiencies of different modified products for polluted water have also been verified by batch experiments. Results proved that Iron oxide modified MMT on the removal efficiency of arsenic significantly increased and the normal titration is better than inverse titration as indicated by the arsenic removal ratios under the same experimental conditions. Based on these results, the iron oxide modified MMT by normal titration procedure is a promising material for removal arsenic in wastewaters.