Graphitic　carbon　nitride　(g-CN)　nanosheets　(CNNs)　with　an　ultra-high　quantum　yield　(80.1%)　ultraviolet　fluorescence　(FL)　were　prepared.　The　effects　of　the　lateral　size　and　the　polymerization　temperature　on　the　optical　properties　of　CNNs　have　been　studied.　The　ultraviolet　FL　was　proved　to　have　originated　from　the　isolated　melem　units　according　to　the　density　functional　theory　calculation　and　mass　spectra.　The　obtained　CNNs　are　further　used　as　a　pH　probe　due　to　the　dependence　of　the　FL　signal　on　the　pH　of　the　solution.