Ruthenium/activated charcoal (Ru/AC) treated with synthesized nano-ZnO (n-ZnO) is used for the first time as a novel composite catalyst in one-step self-condensation of acetone (DMK) to methyl isobutyl ketone in the gas phase. The DMK self-condensation was performed under atmospheric pressure, in a tubular glass fixed-bed microreactor, under DMK and H2 continuous flow at temperature in the range of 523 to 648 K. Addition of n-ZnO to Ru/AC resulted in a pronounced increase in the degree of dispersion of Ru and in the acidic/basic sites concentration ratio. For the one-step synthesis of MIBK at 623 K, the composite catalyst with 2.5 wt % Ru loading was an active and selective bi-functional composite catalyst with balanced acid/base and hydrogenation properties. At 523 K, isopropyl alcohol, product of DMK-direct hydrogenation, was produced in high selectivity for instantly investigated composite catalyst. The catalytic performance depended on the composite catalyst identity, DMK flow rate, H2 flow rate, and reaction temperature.