Abstract: Rheumatoid arthritis (RA) is a systemic chronic inflammatory disease with synovitis and pannus formation as its basic pathologic features. Immune mediated inflammation is the core event in the occurrence and development of RA, but the inflammatory mechanism in RA pathogenesis remains unclear and needs more research to be illustrated. T cells, B cells, proinflammatory cytokine network and chemokines were confirmed to be involved in the process. In addition, the cells related to the structure of bone and joint, such as synovial cells, osteoblasts and osteoclasts, also participate in the inflammation progression of RA acting as the effector cells of immune regulation. The severity of inflammation of RA is closely related to disease activity. There are many kinds of tools for the assessment of disease activity of RA. The rationale use and optimization of these assessment tools will be helpful to make the treatment decision and improve the prognosis of RA.