Background: To explore the effects of β-D-mannuronic acid (M2000) on levels of Th17, regulatory T (Treg) cellsand their related cytokines in patients with ankylosing spondylitis (AS).Methods: 30 AS patients and 15 age and sex-matched healthy individuals were enrolled in this study. The fre-quencies of Th17 and Treg cells and serum levels of related cytokines were measured by flow cytometry analysisand ELISA respectively, before (baseline) and 3 months after M2000 therapy.Results: Significantly higher baseline Th17 cells and serum IL-17, TNF-α and IL-6 were observed in AS patientsthan in normal controls, whereas baseline levels of Treg cells and serum IL-10 were not significantly differentbetween AS patients and healthy controls. After M2000 therapy, frequencies of Th17 and serum levels of IL-17and IL-6 significantly decreased in AS patients. The frequencies of Treg cells and serum level of IL-10 were notsignificantly changed, in comparison to before therapy. Moreover, the correlation analysis showed that fre-quencies of Th17 and levels of IL-17, TNF-α and IL-6 were positively correlated with Bath Ankylosing SpondylitisDisease Activity Index (BASDAI) and Bath Ankylosing Spondylitis Functional Index (BASFI) scores, whereas Tregcells were revealed to be negatively correlated with BASDAI and BASFI scores.Conclusions: It can be concluded that the oral administration of M2000 as a novel NSAID with the im-munosuppressive property that down-regulates Th17 and Th17-related cytokines and facilitates the correction ofthe Th17/Treg imbalance can be effective in the process of AS treatment.