Background : Rheumathoid arthritis is a systemic autoimmune disorder, characterized by erosive arthritis in synovial joints and chronic symmetrical which causes severe dysfunction and disability. People used Siwalan (Borassus flabellifer L.) for antidiabetogenic, antipyretic, antiinflamation, immunosupressive, antibecterial and fertility for a man. Objective: to determine the anti-rheumatoid arthritis activity of the water fraction fruit siwalan in mice models Rheumathoid Arthritis.Methods: Mice induced intraplantar with Complete Freund’s adjuvants on the right foot with a dose of 0.25 ml / animal. Grouping animals were divided into 5 treatment groups, the control group (CFA), the comparison group (methylprednisolone), water fraction group I (dose 22.82 mg / kg), the water fraction II (dose 89.1 mg / kg). Metilprednisolone and water fraction for all dose are given for 20 days. Data were taken on days 1, 5, 10, 15 and 21. The parameters are swelling volume, ankle diameter, scoring macroscopic and back limb histological using Hematoxyllin-eosin staining.Results: Results of data processing using one-way ANOVA on the 15th day of the swelling volume, there were significant differences (p <0.05) between the control group and comparison. There is a trend toward reduction in the swelling volume of water fraction group dose I (0,054 ml ± 0,0056) and dose II (0,055 ml ± 0,0028). In the rat foot diameter measurement the 15th day there are significant differences (95% confidence level) between the control group with the comparison group, the water fraction dose I andIII. And there were no significant differences between the comparison groups with all water fraction group. This means that all water fraction group and the comparison group had the effect of decreasing the diameter of the ankle as well. For data analysis macroscopic scoring on the 15th day there are significant differences (Asymp. Sig. <0.05) between the control group with the comparison group, the water fraction dose II. There were no significant differences between the comparison groups with the water fraction dose group I (22.28 mg / kb mm) and II (89.1 mg / kg bw).Conclusion: Water fraction of siwalan has an anti-rheumathoid arthrithis in mouse models of rheumatoid arthritis induced by CFA.