Background and Purpose: Acute unilateral ptosis in Emergency department(ED) without other focal neurologic impairment isn’t uncommon chief compliant. From internal carotid artery dissection presented with Horner’s syndrome to Posterior communicating(PCOM)aneurysm caused oculomotor nerve (CN III) palsy , our job as ED doctor is to find critical illness. Afterwards, the efficient algorithm for differential diagnosis of acute unilateral ptosis is important. Methods: We use key words” unilat-eral ptosis” and “diagnosis” in PubMed and total 44 full text papers in recent 5 years, which we concluded practical results and finally 25 papers as reference for this article. Results: The eyelid position of ptosis pattern is vary with CN III palsy and Horner’s syndrome. Pupillary involvement requires neuroimaging for the presence of an aneu-rysm that may be compressing the nerve which causing CN III palsy. Horner’s syn-drome only do not appear EOM limitation or diplopia. Beware of bulbar symptoms such as shortness of breath, dysphagia, or dysarthria as these can indicate an impend-ing myasthenic crisis(MG crisis). Conclusion: We can use fast algorithm to find emergent illness of acute unilateral ptosis. By examination of eyelid position, pupillary examination, and extraocular motility(EOM), we may find the possibility of Horner’s syndrome or CN III palsy. Timely treatment can initiation after confirmed image done.