The study investigates the use of copper phosphate nanostructures as catalysts for the direct oxidation of methane (CH4) to formaldehyde (HCHO) using molecular oxygen (O2) as the sole oxidant. Various crystalline copper phosphates with different Cu coordination geometries and Cu/P ratios were synthesized and tested in a fixed-bed flow reactor. The results show that among the investigated catalysts, monoclinic Cu3P2O7, which has a Cu/P ratio of 1/1, exhibited the highest HCHO yield. The catalytic activity of Cu3P2O7 was enhanced by using Cu(NO3)2·3H2O as the copper source, leading to higher HCHO yields compared to other metal phosphates, FePO4, and BiPO4. Mechanistic studies, including catalyst effects, kinetics, isotope labeling, and pulse reaction experiments, revealed that surface lattice oxygen species of Cu3P2O7 likely react with CH4 to produce HCHO as the primary product. The weak basicity of Cu3P2O7 and the presence of redox-active Lewis acidic Cu2+ sites are crucial for C-H activation and suppression of overoxidation to CO2, respectively. Density functional theory (DFT) calculations indicated that the vacancy formation energies at oxygen sites in β-Cu3P2O7, formed by the phase transition of α-Cu3P2O7 under catalytic conditions, are lower than those in α-Cu3P2O7, contributing to the high catalytic performance and durability of Cu3P2O7 for CH4 oxidation to HCHO.The study investigates the use of copper phosphate nanostructures as catalysts for the direct oxidation of methane (CH4) to formaldehyde (HCHO) using molecular oxygen (O2) as the sole oxidant. Various crystalline copper phosphates with different Cu coordination geometries and Cu/P ratios were synthesized and tested in a fixed-bed flow reactor. The results show that among the investigated catalysts, monoclinic Cu3P2O7, which has a Cu/P ratio of 1/1, exhibited the highest HCHO yield. The catalytic activity of Cu3P2O7 was enhanced by using Cu(NO3)2·3H2O as the copper source, leading to higher HCHO yields compared to other metal phosphates, FePO4, and BiPO4. Mechanistic studies, including catalyst effects, kinetics, isotope labeling, and pulse reaction experiments, revealed that surface lattice oxygen species of Cu3P2O7 likely react with CH4 to produce HCHO as the primary product. The weak basicity of Cu3P2O7 and the presence of redox-active Lewis acidic Cu2+ sites are crucial for C-H activation and suppression of overoxidation to CO2, respectively. Density functional theory (DFT) calculations indicated that the vacancy formation energies at oxygen sites in β-Cu3P2O7, formed by the phase transition of α-Cu3P2O7 under catalytic conditions, are lower than those in α-Cu3P2O7, contributing to the high catalytic performance and durability of Cu3P2O7 for CH4 oxidation to HCHO.