Ammonia (NH3) electro-oxidation reaction (AOR) is an important reaction in direct NH3 fuel cells, NH3 electrolyzer, and NH3-based electrochemical sensors. However, its slow kinetics and structure-sensitive properties require specific electrocatalyst designs. In this study, Ir-decorated Pt nanocubes are developed as unique surface engineered model catalysts. For the first time, we find that a trace amount of Ir (less than 2%) could increase the AOR activity of Pt nanocubes by more than twice. Theoretical simulation results also illustrate that the surface-decorated Ir could lower the energy barrier in the rate determining *NH formation step on Pt surfaces, thus increasing activity in accordance with experimental findings. Additionally, Ir and Ni(OH)2-decorated Pt nanocubes can significantly improve the durability. Our findings demonstrate the importance of surface engineering in catalyst synthesis and pave the way of advanced electrocatalyst designs for ammonia oxidation and other catalytic reactions.