Inspired by the excellent activity of platinum in hydrogen evolution reaction (HER) and the good performance of Ni-based compounds in oxygen evolution reaction (OER), a bifunctional electrocatalyst PtNi carbon nanofiber (CNF) is designed and fabricated using electrospinning followed by carbonization. Ultra-small PtNi nanoparticles of several nanometers in size are densely dispersed on every CNF, along with a few larger nanoparticles with sizes of several decades of nanometers. The as-prepared catalysts can be directly used as an electrode and act as high-efficiency materials for water splitting, including HER and OER. For HER activity, the PtNi/CNFs reach 10 mA cm(-2) current density at low overpotentials of 34 mV and exhibit a small Tafel slope of 31 mV dec(-1) in acidic electrolytes of 0.5 M H2SO4, which is close to that of commercial Pt/C (20 wt%) electrocatalytic catalysts. In 1 M KOH solution, the PtNi/CNFs also exhibit excellent HER activity with a low overpotential of 82 mV to achieve a current density of 10 mA cm(-2) and a small Tafel slope of 34 mV dec(-1). Moreover, the PtNi/CNFs also show good activity for OER in alkaline electrolyte of 1 M KOH with a Tafel slope of 159 mV dec(-1) and a small overpotential of 151 mV to reach a current density of 10 mA cm(-2). In addition, the OER performance of the PtNi/CNFs in acid media is also favorable, with a 198 mV dec(-1) Tafel slope. The decent activity of the PtNi/CNFs for water splitting originates from the synergistic effects of using Pt and Ni, large amounts of ultra-small nanoparticles densely dispersed on the CNFs, high conductivity of the support materials and interconnected three-dimensional structures of the carbon nanofiber mats.