Understanding the seasonal variation of arsenic occurrence in shallow groundwater system is one of the challenges and hot spots concerning the research on high arsenic groundwater, which plays an important role in evaluation of safe groundwater availability and remediation of arsenic-affected aquifer. To investigate the seasonal variation of arsenic speciation in the shallow aquifer from the endemic arsenicosis area in the Jianghan plain, groundwater from shallow phreatic aquifer and unconfined aquifer were monitored for arsenic speciation (As(III), As(V) and acid-leacheable particulate As) and water level during pre-monsoon (May), monsoon (July-August) and post-monsoon (November-December) seasons. Arsenic concentration and speciation in groundwater varied temporally: the concentration of As(V) and particulate As were higher, together with higher particulate Fe in most samples during the pre-monsoon seasons; As(V) and particulate As decreased significantly during monsoon seasons, while dissovled As and Fe increased dramatically. Higher As concentrations were associated with an increasing percentage of As(III) in rainy season and a decrease towards the end of dry season, indicating a reductive moblization responding to groundwater level fluctuation. © 2015, Editorial Department of Earth Science-Journal of China University of Geosciences. All right reserved.