Bilateral brachial neuritis is clinically uncommon and accidentally involvement of bilateral phrenic nerves is rarely reported. We experienced a 26 year old man who developed subacute onset of asymmetric bilateral shoulder and arm weakness. The weakness slowly aggravated and finally suffered from dyspnea due to bilateral phrenic nerve palsy. Cervical spine MRI and CSF study showed no abnormality. Viral markers and other serological test showed no specific finding. Electromyographic study showed bilateral brachial axonal polyneuropathy with cervical and upper thoracic polyradiculopathy. And bilateral phrenic nerve conduction study showed no resopnse. He showed no improvement for 10 months after treatment and managed with continuous artificial ventilation. We report a case of idiopathic bilateral brachial neuritis accidentally involving bilateral phrenic nerves.