Developing active, acid-stable and cost-effective electrocatalysts for oxygen evolution reaction (OER) is a primary challenge to directly produce hydrogen from water electrolysis. IrO2 is the best-known catalyst in acid medium due to its good activity and stability, but its too expensive to be used in large amounts. To overcome this economical constraint, the use of low amount of iridium electrocatalyst is required. Recently, ruthenium pyrochlore oxides (A(2)Ru(2)O(7)-delta) as OER catalysts have drawn extensive interests due to reduced content of precious metal relative to RuO2 or IrO2, though their OER catalytic activity still needs to be further improved. In this work, we, for the first time, developed a highly active and stable Y1.85Zn0.15Ru2O7-delta electrocatalyst, the A site doped yttrium ruthenate pyrochlore for OER. The mechanism of A site doped ruthenate pyrochlore improving OER performance is revealed. The partial substitution of Y3+ ions by smaller Zn2+ leads to a formation of oxygen vacancies and mixed valences of Ru (Ru4+ and Ru5+), which, in turn, significantly alter the electronic properties and thus the electrocatalytic activity and electrical conductivity of the developed electrocatalyst for OER. The Y1.85Zn0.15Ru2O7-delta/acetylene black (AB) electrocatalyst exhibits sevenfold higher activity than the IrO2/AB reference catalyst; twofold higher activity than Y2Ru2O7-delta/AB, with a Tafel slope of 36.9 mV dec(-1); and higher stability than IrO2/AB in acidic media. Using a home-made proton exchange membrane electrolyser device, a high cell performance is achieved at 25 degrees C with an electrolysis current density of 0.46 A cm(-2), confirming a promising prospect of Y1.85Zn0.15Ru2O7-delta electrocatalyst for practical water electrolysis applications.