Nowadays, it is necessary and challenging to prepare Cu2O in a large scale for various applications such as catalysis due to its excellent properties. Here, gram-scale Cu2O with nm size is successfully prepared using a simple liquid-phase reduction method at 25 degrees C. The amount of NaOH is found to be the key factor to determine the particle size of Cu2O by modifying the complexation and reduction reactions. The obtained ultra-fine Cu2O exhibits high performance of >95% efficiency for removing high-concentration (3000 ppm) ozone at 25 degrees C and even at a high relative humidity (RH) of 90% for more than 8 h. Furthermore, the Cu2O nanoparticles are coated onto an aluminium honeycomb substrate to form a monolithic catalyst, which shows high ozone removal efficiency of >99% in dry air and >97% in 90% RH for >10 h at a space velocity of 8000 h(-1). The high performance could be attributed to the enhanced release of the ozone decomposition intermediate by the small size of Cu2O, as verified by O-2 temperature-programmed desorption and X-ray photoelectron spectroscopy. All these results show the industrial promise of the large scale synthesis of ultrafine Cu2O applicable for high-performance ozone removal.