In this study, taking Ni enriched water hyacinth (WH, Eichhornia crassipes) as an example, we prepared porous Ni-N doped carbon material (WHPC@Ni) with high supercapacitive performance via fast pyrolysis and KOH activation of WH biomass. The as-prepared WHPC@Ni exhibited an excellent capacitance (552 F g(-1) in 6 M KOH) and high stability (97.5% of capacitance remained after 10 000 cycles), which is much higher than its counterpart WHPC-Ni (derived from WH postadsorbed Ni) and ranks it as one of the best reported biomass-based carbonaceous materials. Multiple characterizations and control experiments showed that the small and monodispersed NiO nanoparticles (similar to 13 nm) in WHPC@Ni played an important role in the enhancement of capacitance. The formation of small NiO nanoparticles may be ascribed to the confinement of organizations of WH (e.g., vessel and cell wall) during the assimilation of Ni, whereas the postadsorbed Ni tended to aggregate during thermochemical treatment (25-85 nm).