The treatment outcome of patients with relapsed or refractory Diffuse Large B-cell Lymphoma (DLBCL) are not satisfactory especially for those not suitable for transplantation. One of main reason is chemotherapy resistance. The investigators conducted this study to evaluate the efficacy of Chidamide combined with R-GDP(rituximab/gemcitabine/dexamethasone/cisplatin) in treating patients with relapsed or refractory Diffuse Large B-cell Lymphoma (DLBCL) not suitable for transplantation. Chidamide is a novel benzamide type of subtype-selective histone deacetylase (HDAC) inhibitor. It has been approved by China Food and Drug Administration (CFDA) for treatment of relapsed or refractory peripheral T-cell lymphoma (PTCL) in Chinese population. The investigators' pre-clinical data suggested that this agent might be also efficient in the treatment of relapsed/refractory B cell lymphoma. In this open-label, non-randomized, phase II study, the investigators aimed to observe the efficacy and safety of chidamide combined with R-GDP in patients with relapsed or refractory Diffuse Large B-cell Lymphoma (DLBCL) not suitable for transplantation.