In diagnosis of arthritis, severity level of synovial inflammation is one of important information to discriminate arthritis types. In this study, we measured the visible absorption spectra of synovial fluid from arthritis patients and found a stronger absorption band between 380 to 450 nm. Then, the measured optical density (OD) of synovial fluid at 420 nm (OD420) was used to estimate severity level of synovial inflammation and compare with clinical classification of inflammation synovial fluid by counting white blood cells (WBCs) number of synovial fluid. In our experimental result, OD420 of synovial fluid concomitantly increased with the severity level of synovial inflammatory. Therefore, we suggest the optical detection of OD420 of synovial fluid can be an indicator to determine severity level of synovial inflammation.