Here in a systematic, accurate and reliable method, Head-Space Gas Chromatography-Mass Spectrometry (HS-GC/MS) was developed to determine blood carboxyhemoglobin (COHb%), in order to investigate deaths related to CO exposure especially involving blood and hepatic tissues. Using a column packed with molecular sieve, COHb levels were quantified down to 0.2% in small blood samples quickly and showed good reproducibility with RSD of the COHb <1%. COHb% in hepatic samples stored at different temperatures (-20 °C for 1-2 years, 0 °C, and 18 °C for two months) can be determined even when the samples are decomposed. The 3-min procedure requires only 0.25 mL of blood sample or 1.0 g of hepatic tissue each time. The technique has a clear advantage over other methods such as UV spectrophotometry.