Benzodiazepines are widely used psychoactive drugs, and have been detected in several clinical cases of accidental exposure and suicide. It was reported that benzodiazepine concentration was changed in post-mortem blood. However, there is no concrete evidence to reasonably explain why benzodiazepine concentration in post-mortem blood cannot be accurately determined. In this study, we showed that the concentrations of almost all types of benzodiazepines examined were significantly decreased in the presence of hemoglobin (Hb) in vitro. In particular, bromazepam was hardly recovered in its intact form. We detected bromazepam metabolites with Hb by liquid chromatography quadrupole time-of-flight mass spectrometry (LC-Q/TOF-MS). The mass spectra showed that bromazepam was metabolized into 3-hydroxybromazepam. Our results suggest that 3-hydroxybromazepam was formed via the Fenton reaction with the divalent iron ion in Hb. Furthermore, 3-hydroxybromazepam was also detected in post-mortem blood of autopsied subjects who intentionally ingested bromazepam, and its concentration increased with time after death. These results suggest that 3-hydroxybromazepam is a potential biomarker of bromazepam poisoning to estimate the amount of bromazepam ingested.