Lipomas are the most common benign neoplasm and usually arise in the subcutaneous tissue. The trunk and proximal limbs are the most frequent sites. Mediastinal lipoma is an uncommon benign neoplasm of mediastinum(1-5% of all lipomas). They are most often located in the anterior mediastinum. They can occur singly or multiply in the mediastinum and can stimulate cardiomegaly or pleural effusion on chest X-ray. They are usually well circumscribed and encapsulated but can grow to be quite large: greater than 20㎝ and 4㎏ in size. A 13-year-old boy was asymptomatic and coincidentally found to have a large soft tissue density in chest X-ray. Computer tomography showed an anterior mediastinal mass consistent of a fat density and vascular structure. Thymus, in the right upper lobe showed mass effect. The mass was surgically excised and pathologically confirmed lipoma. We hereby present the case with review of literature.