Ruthenium nanoparticles (10–40 nm) were synthesized by the reduction of aqueous ruthenium trichloride in aqueous sodium borohydride by the water-in-oil (w/o) reverse microemulsion method at atmospheric temperature. The synthesized Ru nanoparticles (Ru content 1–5 wt%) were dispersed on γ-alumina of high surface area by a high speed mechanical stirrer. The nanocatalysts (Ru/γ-Al2O3) were characterized by transmission electron microscopy, X-ray diffraction, N2-physisorption, BET surface area, hydrogen chemisorption and thermogravimetric analysis (TG/DTA). The activity of the nanocatalyst was tested in the gas–liquid phase hydrogenation reaction of benzene to cyclohexane at 40–100 °C and 20 bar hydrogen pressure. The activities of the catalysts were studied by varying different parameters such as Ru loading (wt% of Ru), reaction temperatures, catalyst recycling, thiophene (catalyst poison) concentration, stirring speed and metal nanoparticles size etc. The results showed that maximum activity (100 % conversion of benzene) was obtained at 80 °C and 20 bar pressure with 4 wt% (Ru content) catalyst. The activation energy and TOF were found to be 27.9 kJ mol −1 and 1416.8 h−1, respectively, which indicated higher activity of Ru/γ-Al2O3 catalyst than conventional catalyst for the hydrogenation of benzene to cyclohexane.