Octahedral nickel ferrite oxide (NiFe2O4) nanocrystals with average sizes of 81, 69, 63 and 46 nm were fabricated using hexamethylenetetramine as adscititious alkali via a facile hydrothermal route at various temperatures. The formation mechanism of octahedral nickel ferrite oxide nanocrystals was discussed in detail. Interestingly, the nanocrystalline size decreased with the increase in the hydrothermal reaction temperature. We studied the influence of hydrothermal temperatures on the evolution of the nanocrystalline and analyzed the relationship between the sizes of the nanocrystalline and their capacitive properties. Compared to the large-sized counterpart (81, 69 and 63 nm), the small-sized nanocrystals (46 nm) presented a maximum specific capacitance (562.1 F g(-1)) and remarkable cycling stability (80.3% capacity retention after 1500 cycles) at 4 A g(-1). The excellent performance of the NiFe2O4 nanocrystals (46 nm) was mainly attributed to the unique octahedral nanostructures with a small size (fully exposing more electroactive sites and providing more sufficient expressways for rapid charge transfer) and their compositional advantages of nickel and cobalt (multiple oxidation states for redox reactions and relatively desirable electroconductivity). More remarkably, an asymmetric supercapacitor composed of NiFe2O4 (as the positive electrode) and activated carbon (as the negative electrode) displayed an ultrahigh energy density (34.91 Wh kg(-1) at 1100 W kg(-1)) and an advanced cycling stability (84.5% capacity retention after 1000 cycles), which suggested that the decreased crystal size played a pivotal role in size-dependent capacitive performance enhancement.