Masseter muscle rigidity may herald a fulminant malignant hyperthermia reaction or may be an isolated abnormal anesthetic response. It is very hard to differentiate, Significant masseter muscle rigidity is a muscle spasm of at least 30 seconds duration that interferes with mouth opening despite an appropriate succinylcholine dose in a patient who has no temporo-mandibular joint dysfuntion. They thought mild to moderate foims of spasm are tolerable for anesthesia. We suffered incomplete relaxation of masseter muscle after succinylcholine that followed by fulminant malignant hyperthermic reaction. At that time, a little resistance was felt during mouth opening put intubation was done . without difficulty. After this experience, because Caffeine-Halothane contracture test is performed only in limited institutes and dantrolene is not available in our hospital, we decide to hold the anesthetic practice in case of any degree of masseter muscle rigidity.