Abstract：In 1990, the chronic arsenicosis patients due to drinking contaminated groundwater were reported in the Hetao plain of Inner Mongolia, China. At present, it is estimated that more than 410,000 of inhabitants are damaged by arsenic poisoning in Inner Mongolia. We carried out hydrogeological survey, geological survey, medical survey and installation of piped water supply systems for an 8-year period from 1997 to 2004. These surveys revealed the mechanism of the groundwater contamination by arsenic in the Hetao plain as follows: Fe-oxyhydroxide is dissolved by intensifying reducing condition in the aquifers as a result the adsorbed arsenic is released into groundwater. In order to prevent further increase of arsenicosis patients, it is important to stop ingesting contaminated water, and to supply safe drinking water. For that purpose we need to understand the mechanism of arsenic contamination based on geological and hydrogeological conditions of the field.