It is still highly desired to develop efficient, resource-abundant and inexpensive electrocatalysts to improve the sluggish kinetics of oxygen evolution reaction (OER) in electrochemical water splitting systems. In this work, the large-area ultrathin (2.52 nm thick) Ce-doped La2O3nanofilms were developed via a facile and reliable ionic layer epitaxy method with different Ce content. The ultrathin Ce-doped La2O3nanofilm with optimum composition of La1.22Ce0.78O3exhibited an excellent OER performance with a very low overpotential of 221 mV at 10 mA cm-2and a small Tafel slope of 33.7 mV dec-1. A remarkable high mass activity of 6263.2 A g-1was also obtained from ultrathin La1.22Ce0.78O3nanofilm at the overpotential of 221 mV. Such a high mass activity was three orders of magnitude higher than state-of-the-art commercial IrO2powders (3.8 A g-1) and more than 30 times higher than La2O3nanofilm (196.7 A g-1) without Ce doping at the same overpotential. This high mass activity was even significantly higher than other recently reported typical OER catalysts. The substantial OER performance gain by the Ce doping was attributed to the improved conductivity and electrochemical active surface areas of nanofilms as a result of favorable tuning on the charge transfer and electronic structures. This work provides a promising approach to develop high-performance two-dimensional (2D) electrocatalysts by effective heteroatom doping strategy.