Ultrathin (∼10 nm) InN ion selective field effect transistors (ISFETs) show a current variation ratio of 3.5 % per pH decade with a response time of less than 10 s. When the ISFET is employed as an electrolyte FET, the current variation of 18 % was measured as the gate bias changes from zero to 0.3 V given a drain-source voltage of 0.1 V. The high current (resistance) variation ratio is attributed to the ultrathin epilayer and an unusual phenomenon of intrinsic strong electron accumulation on InN surface, which enables a chemical/biological sensor with high sensitivity and resolution and permits detection of a slight concentration variation of the electrolyte. The pH response measurement of 10-nm-thick InN ISFETs investigated was performed in an aqueous solution titrated with diluted NaOH and HCl. The Helmholtz potential built at the electrolyte-InN interface is governed by direct adsorption of H+ ions at the surface metal oxides, modulating the channel current of the InN ISFETs. The channel current monotonically decreases as the pH value of an aqueous solution increases from 2 to 10. The sensitivity and resolution were found to be 58.3 mV per decade and 0.02 pH change, respectively. Besides, the detection of DNA hybridization was further performed after the InN surface was modified with MPTMS and probe DNA. A complementary target DNA solution of 100 nM led to a current decrease of approximate 6 uA, corresponding to the current variation of 0.74 %. The hybridization between negatively charged complementary DNA and the immobilized probe DNA caused the depletion of carriers at the InN surface, suppressing the channel current. The functionalized InN ISFETs are suitable for genetic analysis in clinical diagnostics without any labeling reagent. Such an InN-based sensor is appealing in the regime of chemical and biological sensing applications.