Primary cardiac lymphomas are extremely rare, accounting for 1.3% of primary cardiac tumors and 0.5% of extranodal lymphomas. The clinical manifestations may be variable and are attributed to its location, the presence of congestive heart failure, pericardial effusion and arrhythmia. Here, we report a case of primary cardiac lymphoma presenting with atrioventricular block. A 55 year-old man had experienced discomfort in chest with unexplained dyspnea and night sweating. His initial EKG revealed normal sinus rhythm and first degree atrioventricular block. When chest discomfort and dyspnea worsened, his EKG changed to sinus bradycardia with secondary atrioventricular block (Mobitz type I). A CT scan confirmed cardiac mass(about 7cm) and excisional biopsy was done. The pathologic finding confirmed diffuse large B-cell lymphoma. The patient was treated with R-CHOP regimen chemotherapy. After chemotherapy, EKG changed to normal sinus rhythm and the follow-up CT showed that the cardiac mass had almost completely disappeared.