In the present study, Raman spectral characteristics of methemoglobin (MetHb) induced by sodium nitrite (NaNO2) were investigated. Hemoglobin (Hb) was oxidated to MetHb with NaNO2, the Raman spectral specific changes of MetHb was studied by determining the Raman spectral changes of methemoglobin/total hemoglobin of different ratios, and the Raman intensities of methemoglobin/total hemoglobin of different ratios at 1 586, 1 605 and 1 637 cm-1 were linearly fitted to realize its quantitative detection. The results show that the completely oxidized MetHb can be obtained when the molar ratio of NaNO2 to Hb is 3.5∶1 whose Raman characteristic peaks are at around 499, 1 340, 1 562 and 1 622 cm-1, and that the linear fitting correlation coefficients R2 of the Raman intensities of methemoglobin/total hemoglobin of different ratios at 1 586, 1 605 and 1 637 cm-1 are 0.972 84, 0.997 97 and 0.991 26 respectively, which shows a good linear relationship. This study indicates that the Raman spectrums of MetHb induced by NaNO2 have characteristic differences when compared with normal Hb, that the locations and intensities of Raman characteristic peaks change correspondingly with the alterations of the ratios of methemoglobin/total hemoglobin, and that there are linear correlations between the ratios and their corresponding Raman intensities, which would provide theoretical bases for the clinical Raman spectral detection and quantitative study of methemoglobinemia.