Hierarchically porous Pd/NiO composites were successfully prepared through a hydrothermal and in-situ reduction protocol. NiO nanoparticles less than 10 nm aggregated together to form irregular NiO nanosheets and finally composed the flower-like NiO sphere. When Pd nanoparticles were evenly dispersed and loaded, the resulting materials showed high catalytic activity for low temperature CO oxidation especially under ambient conditions. For 3 wt% Pd loaded catalyst, T50 for 1 vol% CO conversion was as low as -15 degrees C under a space velocity of 15 000 ml h(-1) gcat(-1). The kinetic study revealed the presence of water molecules can drastically lower the activation energy and finally promote the CO oxidation process. (C) 2017 Elsevier B.V. All rights reserved.