Abstract: Three xanthan oligosaccharides ( XGO-A, XGO-B and XGO-C) were prepared by oxidative degradation in alkaline condition.The products were characterized by FT- IR, and the molecular weights of the products determined by GPC method were 3600, 7500 and 10100, respectively.The pyruvate acid contents of the three xanthan oligosaccharides were 5.7%, 5.3% and 4.8%, respectively, and reducing sugar contents were 23.7%, 22.5% and 21.1%, respectively.The antibacterial activity of three xanthan oligosaccharides against the X.campestris was investigated by the inhibition zone, minimal inhibitory concentration ( MIC) and the influence on the growth of X. campestris.Effects of xanthan oligosaccharides on electric conductivity of liquid medium of X.campestris were also determined.The results indicated that XGO-B exhibited higher antibacterial activity than XGO-A and XGO-C. It meant that the antibacterial effect of xanthan oligosaccharides may be related to their molecular weight.The effect of xanthan oligosaccharides on the cell membrane permeability of X.campestris became stronger when the molecular weight of xanthan oligosaccharides increased.It indicated that the damage of cell membrane of X.campestris is one of the antibacterial mechanisms to X.campestris.