This paper reports ion sensitive field effect transistor (IS-FET) with graphene/ionophore hybrid membrane for phosphate detection. CVD graphene is used as a sensing material because the electrical property of graphene can be easily changed by a surrounding condition. In the experiment, a shift of Dirac point is measured to check the concentration of the ions in the water. Since graphene does not have selectivity for the specific ions, phosphate selective membrane is used for the selectivity. The membrane is synthesized by molecular imprinted polymer. To check the selectivity of the graphene sensor coated with the selective membrane, the IS-FET is characterized with several solutions, which have different ions and concentrations. The sensor response is measured by a semiconductor analyzer. Four different ion solutions are used for the measurement (phosphate, sulfate, chloride, and nitrate). Each solution has four different concentrations (0, 0.1, 1 and 10 mg/L). The Dirac point shift from − 0.4 to − 0.2 V as increasing the concentration. The detection limit and response time are 0.1 mg/L and 10 s, respectively.