Method of quantitative analysis through latex-state 13 C NMR spectroscopy was established for in situ determination of epoxy group content of epoxidized natural rubber in latex stage. The epoxidized natural rubber latex was prepared by epoxidation of deproteinized natural rubber with freshly prepared peracetic acid in latex stage. The resulting epoxidized deproteinized natural rubber (EDPNR) latex was characterized through latex-state 13 C NMR spectroscopy. Chemical shift values of signals of latex-state 13 C NMR spectrum for EDPNR were similar to those of solution-state 13 C NMR spectrum for EDPNR. Resolution of latex-state 13 C NMR spectrum was gradually improved as temperature for the nuclear magnetic resonance (NMR) measurement increased to 70°C. Signal-to-noise ratio of latex-state 13 C NMR measurement was similar to that of solution-state 13 C NMR measurement at temperature above 50°C. The epoxy group content determined through latex-state NMR spectroscopy was proved to be the same as that determined through solution-state NMR spectroscopy.