Isaacs syndrome is rare disorder with peripheral nerve hyperexcitability syndromes with acquired neuromyotonia in childhood. We present a 13-year-old girl with muscle stiffness and neuromyotonia diagnosed Isaac syndrome with spontaneous discharge potentials on motor unit in electromyography and the diagnosis supported by the presence of antinuclear antibodies. A successful treatment was obtained using low-dose carbamazepine. Cause of Isaacs syndrome is unknown, generally thought to be an autoimmune etiology with voltage-gated potassium channelopathy; it sometimes occurs as a paraneoplastic syndrome. Early use of electromyography has critical role in the differential diagnosis with certain muscle disorders and peripheral nerve hyperexcitability syndromes.