A　novel　light-induced　reversible　self-assembly　(LIRSA)　system　is　based　on　the　reversible　photodimerization　and　photocleavage　of　coumarin　groups　on　the　surface　of　gold　nanoparticles　(AuNPs)　in　THF　solution.　Facilitated　by　coumarin　groups,　light　irradiation　at　365nm　triggers　the　stable　assembly　of　monodisperse　AuNPs;　the　resulting　self-assembly　system　can　be　disassembled　back　to　the　disassembled　state　by　a　relatively　short　exposure　to　benign　UV　light.　The　reversible　self-assembly　cycle　can　be　repeated　4times.　A　specific　concentration　range　of　coumarin　ligand　and　the　THF　solvent　were　identified　to　be　the　two　predominant　factors　that　contribute　to　the　LIRSA　of　AuNPs.　This　is　the　first　successful　application　of　reversible　photodimerization　based　on　a　coumarin　derivative　in　the　field　of　AuNP　LIRSA.　This　LIRSA　system　may　provide　unique　opportunities　for　the　photoregulated　synthesis　of　many　adjustable　nanostructures　and　devices.