Malignant hyperthermia is a catastrophic, hypermetabolic syndrome that arises in susceptible individuals when they are exposed to certain inhalational anesthetics or muscle relaxants. It is characterized by hyperthermia, tachycardia, acidosis, and muscle rigidity. It has been noted that the majority of cases of malignant hyperthermia are fatal unless early diagnosis and treatment are performed. We experienced a 24 year old male Malignant hyperthermia presented for orthognathic surgery under $O_2-N_2O$-sevoflurane anesthesia without succinylcholine. Two half hours after induction, tachycardia developed and was followed by unstable blood pressure and hyperpyrexia. Anesthesia was terminated and vigorous emergency treatment was attempted. The patient was treated by the intravenous administration of dantrolene sodium. The diagnosis of an acute malignant hyperthermia reaction by clinical criteria can be difficult because of the nonspecific nature and variable incidence of many of the clinical signs and laboratory findings. So the malignant hyperthermia clinical grading scale is recommended for use as an aid to the objective definition of this disease. This clinical grading system provides a new and comprehensive clinical case definition for the malignant hyperthermia syndrome. We recently encountered a case of delayed malignant hyperthermia during sevoflurane anesthesia that was successfully treated by the intravenous administration of dantrolene sodium. In conclusion, exposure to sevoflurane should be avoided in patients thought to be susceprible to malignant hyperthermia.