Isolated hemimegalencephaly (HME) is a rare congenital malformation of brain development, remarkable for its extreme asymmetry, and is characterized by the overgrowth of part or whole hemisphere. The enlarged hemisphere is manifested by hamartomatous characteristics with a dysplastic cell array of atypical morphology. Traditionally the genetic theories regarding the pathogenesis of HME are considered due to disturbance in cell signaling during neuroblast migration, cell differentiation, and proliferation, pattering, and symmetry. HME can present as isolated or associated with several neurocutaneous syndromes. The clinical picture varies depending on the severity of the malformation; however, HME patients typically exhibit refractory epilepsy, macrocephaly, colpocephaly, global developmental delay, intellectual disability, hemibody hypertrophy, and hemiparesis. Early diagnosis is crucial because despite neuroimaging and pathologic evidence, hemimegalencephaly sometimes still is unrecognized. Also, misdiagnosed as obstructive hydrocephalus or cerebral neoplasm can lead to unnecessary surgical procedures. Although hemispherectomy has high morbidity, it is recommended early for patients with severe, intractable epilepsy. We report a diagnosed case of a 22-month-old boy hemimegalencephaly who presented with seizures attack and was successfully treated with antiepileptic medications.