Nanowire field-effect transistors are suited to study the activity of biomolecules in bionanotechnology. The changes of biomolecules process are efficiently affected the charge at the nanowire surface; thus, the electrical characterization of NW-FET is changed. Although NW-FET is a well-known device in bioapplications, however, the intrinsic electrical characterization of NW-FET effect on real electrical measurement is not well studied. We present herein a novel measurement method to avoid errors in electrical characteristic of nanowire field-effect transistors. A physical model is developed to explore the effect of the leakage current, which is influenced by the charging effect of an equivalent capacitor in a NW-FET. We also present a sloped-gate voltage method to reduce the effect of equivalent capacitor in air, liquid, and phosphate buffered solution. The application of the sloped-gate voltage method significantly increases the stability of electrical characterization measurements. This method can also be easily applied to biosensing experiments.