Primary mediastinal seminoma is a rare germ cell tumor that usually occurs in young men. Recently we experienced two cases of primary mediastinal seminoma with CT scan. CT findings were a large anterior mediastinal mass which grows mainly to the left and displaces left bachiocephalic vein posteriorly, lobulated tumor margins with small spiculations or projections, loss of fat planes between the tumor and surrounding organs and homogeneous density of 40-50 HU in postcontrast study. These CT findings may suggest the possibility of primary mediastinal seminoma before histologic confirmation.