Photocatalytic conversion of CO2 to value-added fuels, a promising route to address the depletion of fossil fuels and concomitant global warming, can be improved by cocatalysts through promoting the electron-hole separation and offering catalytic active sites for the surface reactions. However, the enhancement of the photo catalytic performance is greatly determined by the surface and interface structures of cocatalysts. Herein, for the first time, we demonstrate that the photocatalytic activity in the CO2 reduction can be optimized through crystalline phase design of cocatalysts. In this work, Ru nanocrystals in face-centered cubic (fcc) and hexagonal close-packed (hcp) phases are in-situ grown on C3N4 nanosheets to form different C3N4-Ru hybrid structures, respectively. It was found that the hcp Ru achieved higher average CO and CH4 production rate but lower H-2 production rate in comparison with fcc Ru. As revealed by the experimental characterizations combined with theory simulations, the phase-dependent photocatalytic performance is resulted from the different surface reaction behaviors on the fcc and hcp Ru cocatalysts. The adsorption energy of CO2 molecules on the dominated (10 (1) over bar1) face of hcp Ru is higher than that on the dominated (111) face of fcc Ru. As a result, the stronger interaction between CO2 molecules and the surface of hcp Ru contributes to the enhanced photocatalytic activity and selectivity of C3N4-hcp Ru in reduction of CO2. This work highlights the importance in the crystalline phase engineering in cocatalyst for enhanced CO2 photoreduction.