This study was aimed to investigate the relationship of Ki-67 proliferation index (Ki-67 PI) with non-Hodgkin's lymphoma(NHL) typing and biological behavior, as well as its significance in clinical characters and prognosis of diffuse large B-cell lymphoma(DLBCL). A total of 542 cases of NHL in our hospital from 1st January 2001 to 31st December 2010 were retrospectively analyzed, and Ki-67 PI was all assayed immunohistochemically, and a total of 82 cases of newly-diagnosed DLBCL with more clinical records were investigated. The results indicated that according to the World Health Organization (WHO) histopathological classification of lymphoma, Ki-67 PI was different as classification for NHL subgroups was different. The Ki-67 PI increased with aggressive progression of NHL. The mean Ki-67 PI ranged from 25.5% in indolent lymphoma to 98.4% in very aggressive lymphoma. ROC curve analysis demonstrated that the 50% was the cut-off value distinguishing indolent from aggressive disease. On ROC curve analysis, Ki-67 PI of 75% was found to significantly discriminate patients with DLBCL who had a good or bad prognosis. There was a significant correlation of Ki-67 PI with Ann Arbor stage and LDH level. When the DLBCL cases were divided by Ann Arbor stage and IPI score, the 3-year overall survival (OS) of patients with a low Ki-67 PI (≤ 75%) in the group of Ann Arbor stage III-IV and high LDH level was higher than those with a high Ki-67 PI (> 75%) among the patients with B symptoms and IPI 3.0-5. 3-year OS in those with a low Ki-67 PI (≤ 75%) in the group of Ann Arbor stage III-IV and normal LDH level was higher than those with a high Ki-67 PI (> 75%) among the patients with B symptoms. 3-year OS of patients with a low Ki-67 PI (≤ 75%) in the group at III-IV stage and a high LDH level was higher than those with a high Ki-67 PI (> 75%). It is concluded that a cut-off value of 50% can be helpful to differentiate indolent from aggressive NHL. In DLBCL, a cut-off value of 75% can distinguish patients with a good or bad prognosis when combined with other prognostic factors, i.e. B symptoms, Ann Arbor stage, IPI score and lactate dehydrogenase (LDH) level.