A now technique to fabricate planar solid-state reference electrodes for ISFET sensors is reported. Agarose powder was introduced to gel the KC1 saturated electrolyte to serve both as a polymer-stabilized internal electrolyte and an ion-diffusion membrane for Ti/Pd/Ag/AgC1 planar electrode. It enables the integration of a reference electrode on the sensor chip such as ISFET for ion sensing measurement. This new planar integrated reference electrode has not only eliminated the fabrication problems associated with the filling of the inner reference liquid electrolyte into a miniature micromachined cavity, but also has many excellent performance characteristics with respect to the reference potential stability and its insensitivity to the changes of pH values and C1(-) ion. concentrations in the sample solutions under test. Calibrated against the standard Ag/AgC1 reference electrode the new planar reference electrode shows its cell potential variation less than 1 mV over 30 min; less than 2 mV variation over pH values ranging from 4 to 10; and almost insensitive (approx. 0.02 similar to 0.25 mV/pKC1) to changes in C1(-) ion concentration. The reference electrode also shows a very small offset voltage of 0.45 mV and reproducible to within 0.5 mV among the batch fabricated electrodes.