Abstract: Objective The changes of blood lactate dehydrogenase(LDH) of cyanide poisoning cases were carefully observed to provide the orientation for the diagnosis and grading of cyanide poisoning. Methods A total of 6 cases with accidental hydrogen cyanide poisoning were studied and their time-course changes of blood lactate dehydrogenase were recorded till 108 hours. Results Lactate dehydrogenase level of hydrogen cyanide poisoning patients was above 300 U/L and recovered slowly, while that of exposed persons was less than 300 U/L and recovered rapidly. Conclusion It suggested that the lactic dehydrogenase was useful for the diagnosis at early stage.