Abstract: FLB5M5 is a humanized monoclonal antibody with three mutated amino acids in the CDRs relative to its parental mouse anti-IL-20 monoclonal antibody 7E and five mutated amino acids of the light-chain framework region relative to the amino acids of the light-chain framework region of human V?2. FLB5M5 not only retains binding specificity toward IL-20 but also has a better binding affinity than 7E for IL-20. FLB5M5 is also less immunogenic than 7E to the human host in clinical application. A mutation in the light chain CDR to tyrosine increases binding affinity to IL-20. A method for treating rheumatoid arthritis using FLB5M5 is also disclosed.