Abstract: Reducing the mean transverse energy (MTE) of photoemitted electrons is essential for generating low-emittance, high-brightness electron beams, and is of particular importance for advanced accelerator-based applications such as X-ray free-electron lasers (XFELs), ultrafast electron diffraction (UED), and ultrafast electron microscopy (UEM). Metallic photocathodes have been widely employed in high-brightness electron sources owing to their excellent stability, high electric-field tolerance, and relatively mature fabrication processes. In this work, angle-resolved photoemission spectroscopy (ARPES) was employed to systematically investigate the MTE characteristics of single-crystal copper photocathodes with different crystallographic orientations. The influence of surface treatment procedures on the transverse energy distribution of photoemitted electrons was also examined. Under identical excitation conditions, pronounced differences in MTE were observed among different copper crystal facets, with the Cu(110) surface exhibiting the lowest MTE. In addition, single-crystal copper samples treated by Ar-ion sputtering followed by annealing exhibited lower MTE values than samples subjected to annealing alone. These results demonstrate that the MTE of single-crystal copper photocathodes is not determined solely by excess energy. Crystallographic orientation and surface condition also affect the transverse momentum distribution of photoemitted electrons, thereby modifying their MTE. The coordinated optimization of crystallographic orientation and surface-treatment processes is expected to further reduce electron-beam emittance and provide experimental support for the design and optimization of high-brightness electron sources.