One of the main challenges of electrochemical water splitting technology is to develop a high performance, low cost oxygen-evolving electrode capable of substituting a noble metal catalyst, Ir or Ru based catalyst. In this work, CoFe2O4 nanoparticles with sub-44 nm-size of a inverse spinel structure for oxygen evolution reaction (OER) were synthesized by the injection of KNO3 and NaOH solution to a preheated CoSO4 and Fe(NO3)3 solution. The synthesis time of CoFe2O4 nanoparticles was controlled to control particle and crystallite size. When the synthesis time was 6 h, CoFe2O4 nanoparticles had high conductivity and electrochemical surface area. The overpotential at current denstiy of 10 mA/cm2 and Tafel slope of CoFe2O4 (6h) were 395 mV and 52 mV/dec, respectively. In addition, the catalyst showed excellent durability for 18 hours at 10 mA/cm2.