We prepared nano zero-valent iron/biochar composites (nZVI/BC) to remove nitrate from groundwater with harmless products. Results show nZVI/BCs derived from different mass ratios of Fe(0) to biochar exhibited different nitrate removal efficiencies, and the mass ratio of 1: 2 developed the nZVI/BC with optimum nitrate removal. The nZVI/BC maintained preferable nitrate removal (75.0%-97.0%) over a wide pH range 2-12. Nitrate removal amount declined due to serious corrosion and clogging of nZVI/BC's surface when initial nitrate concentration exceeded 40 mg/L. The nitrate removal process followed a first-order kinetic reaction. As regards real groundwater, the nZVI/BC removed more nitrate than both pure Fe(0) nanoparticles and biochar. Reaction product analysis revealed 60.1% of removed nitrate selectively became dinitrogen. X-ray photoelectron spectroscopy measurements suggested that nitrate-oxidized Fe(0) resulted in significant increase of magnetite on the surface of nZVI/BC. The redox potential and pH of long duration in reaction changed around -210 mV and 8-9, respectively. These changes facilitated the selective reduction of nitrate to dinitrogen. We also proposed that biochar may provide favorable circumstances for nitrate reduction by directly mediating redox potential, pH and electron transfer, which establishes a possible mechanism for the enhanced removal and selective reduction of nitrate. Our study suggests that nZVI/BC would be a promising alternative for the remediation of nitrate-contaminated groundwater.