The objective of the present study was to evaluate the effect of a mixed extract of three herbs, Panax Notoginseng, Rehmanniae Radix and Acanthopanacis cortex (AIF), for the treatment of horses with experimentally induced osteoarthritis. Twelve healthy male horses were included in this study. Horses were assigned to one of two groups: the AIF group (n=6) or the control group (n=6). Osteoarthritis was induced in all horses by intraarticular injection of sodium monoiodoacetate (0.12 mg/kg). Horses in the AIF group received 3 g of AIF with food daily, and those in the control group received food only. Treatment began on the day of intraarticular injection. Clinical and radiographic evaluations were performed every 2 weeks. At week 12, horses were euthanatized, and postmortem gross pathologic and histologic examinations of the middle carpal joint were performed. There were no significant differences in clinical values between the two groups. Radiographic evaluation revealed that the percentages of narrowness of joint space width in the control group were significantly higher than those in the AIF group (p＜0.02). On gross pathologic examination, the mean total dimensions of articular cartilage erosions and fibrillations in the control group (101.5±41.5 mm²) were significantly wider than those in the AIF group (29.3±39.7 mm²; p＜0.01). On histopathologic evaluation, significantly higher grades of staining intensity and lower empty lacunae (EL) ratios were found in the AIF group (p＜0.03). The present study revealed that AIF had significant disease modifying effects in horses with experimentally induced osteoarthritis.