We herein present a patient with Morgagni hernia which was diagnosed by magnetic resonance imaging (MRI). The patient had a progressively enlarging mass in the right cardiophrenic angle on chest roentgenogram. On computed tomography (CT) scans, the mass was revealed to have fat density and therefore was suspected to be either a lipoma or liposarcoma. MRI clearly demonstrated that the mass shadow was composed of omental fat herniating into the right thorax through the diaphragmatic hiatus. MRI is thus considered to be a useful noninvasive modality for the evaluation of lower anterior mediastinal masses demonstrating fat density on CT.