To explore a new method to dignose methemoglobin, micro-Raman spectroscopy was used to investigate the interaction of sodium nitrite and oxyhemoglobin in aqueous solution, with 514.5 nm exitation. Hemoglobin was obtained from the healthy adult. The results show that the inensity ratio of the oxyhemoglobin marker band 570 cm-1 to the aquomethemoglobin marker band 495 cm-1, and that of low spin state marker band 1586 cm-1 to the high spin state marker band 1555 cm-1 decrease during the interaction. The bands sensitive to the oxidation state shift to higher frequency. This work indicates that micro-Raman spectroscopy could monitor the bioeffects of sodium nitrite on the oxyhemoglobin, and distinguish hemoglobin and aquomethemoglobin. As a new method to detect methemoglobin, micro-Raman spectroscopy may have great potential applications in the diagnosis of methemoglobinemia.