Abstract: The mercuric ion sensor was prepared with immobilized mercuric reductase covalently coupled to porous glass. The mercuric reductase was immobilized on an arylamino derivative of porous glass with azo coupling. The mercuric ion sensor system was assembled by a flow injection analysis (FIA) system. With an enzymatic cycling reaction constructed by mercuric reductase and catalase, concentrations of mercuric ion >0.025 µM (5 ppb) were detectable by this mercuric ion sensor with 0.2 mL of sample solution Thus, this mercuric ion sensor is suitable for mercuric detection in low concentrations using small sample volumes without any pre-treatment of the samples.