The supported Ru clusters with mean sizes ranging from 1.9 to 4.6 nm showed a high activity towards the NH3 decomposition reaction. The structural properties of catalysts were characterized by N-2 adsorption/desorption, X-ray diffraction (XRD) and transmission electron micrograph (TEM). Steady-state reaction kinetics revealed that the apparent activation energy increased with a decrease in Ru particle size and ranged from 79 kJ mol(-1) to 122 kJ mol(-1). The decomposition rate over Ru nanoparticles showed a strong dependency on mean crystallite size and the optimum appeared at d(Ru) = 2.2 nm. The dependencies of reaction rate on partial pressures of NH3 and H-2 were also sensitive to the varying Ru particle size. Experimental data could be well fitted by the Temkin-Pyzhev equation, indicating that the recombinative desorption of surface nitrogen atom acts as the rate-determining step. A compensation effect between the pre-exponential factor (k(0)) and activation energy (E-a ) was quantified.