The DC operation of diamond electrolyte solution gate FETs (SGFETs) and discrimination of complementary and singly mismatched DNAs in solution were demonstrated. The transconductance (gm), which is the sensitivity for detecting the hybridization of DNA on SGFET, was increased from 60 μS/mm to 9.5 mS/mm by miniaturization of FET. Hybridization of target DNA with probe DNA was detected by the gate potential shift caused by the negative charges on DNA immobilized on the channel surface. The change in gate potential caused by the hybridization of complementary DNA was about 25 mV. The ratio of change in gate potential by hybridization of complementary and single-mismatched DNA was 3:1 on diamond SGFET.