Ultrasmall Ru nanoparticles is expected as a potential alternative to Pt for efficient hydrazine oxidation (HzOR). However, preparation of ultrasmall and well-distributed Ru nanoparticles usually suffered from the steps of modification of supports, coordination, reduction with strong reducing reagents (e.g., NaBH4) or pyrolysis, imposing the complexity. Based on the self-reducibility of C-OH group and physical adsorption ability of commercial Ketjen black (KB), we developed an efficient, stable and robust Ru-based electrocatalyst (A-Ru-KB) by coupling impregnation of KB in RuCl3 solution and simple in situ electrochemical activation strategy, which endowed the formation of ultrasmall and well-distributed Ru nanoparticles. Benefiting from an enhanced exposure of Ru sites and the faster mass transport, A-Ru-KB achieved 63.4 and 3.9-fold enhancements of mass activity compared with Pt/C and Ru/C, respectively, accompanied by a ∼144 mV lower onset potential and faster catalytic kinetics than Pt/C. In the hydrazine fuel cell, the open-circuit voltage and maximal mass power density of A-Ru-KB was 130 mV and ∼3.8-fold higher than those of Pt/C, respectively, together with the long-term stability. This work would provide a facile and sustainable approach for large-scale production of other robust metal (electro)catalysts with ultrasmall nanosize for various energy conversion and electrochemical organic synthesis.