Photothermal conversion has shown great potential for the effective use of solar energy. Au nanoparticles (Au NPs) were a satisfactory material for photothermal conversion, mainly due to their surface plasmon resonance (SPR) effect. Herein, the diameter effect of Au NPs on photothermal conversion was explored for generating solar steam. The diameters varied from 3 to 40 nm, and the results showed that the photothermal conversion efficiency was enhanced by increasing the diameters of the Au NPs with constant total superficial areas under a xenon lamp. This study is important for the further exploration of the photothermal effect of Au NPs and their practical applications in large-scale solar steam generation, seawater desalination and sterilization of waste.