Surface plasmon resonance (SPR) and Z-scheme bridge are the most important roles of noble metal nanoparticles for photocatalysis. In this study, the size-dependent role of gold (Au) in Au/g-C3N4/BiOBr system was studied for photocatalytic CO2 reduction and dye degradation. The results revealed that big Au nanoparticles mainly displayed Z-scheme bridge role, and g-C3N4/BiOBr/Au-B showed higher photocatalytic activity than g-C3N4/BiOBr under 380 nm monochromatic light irradiation. Small Au nano particles mainly displayed SPR role, and g-C3N4/BiOBr/Au-S showed higher photocatalytic activity than g-C3N4/BiOBr under 550 nm monochromatic light irradiation. The size-dependent role of Au nano particles for photocatalytic system should be due to the SPR effect of small Au nanoparticles and electron transfer center of big Au nanoparticles. This finding can guide us to prepare high photocatalytic activity noble metal loaded photocatalysts. (C) 2016 Elsevier B.V. All rights reserved.