Abstract: The xanthan gum is the extracellular polysaccharide secreted by xanthomonas campestris from wild rape. It can be produced by fermentation with starch or glucose as the carbon source. Polysaccharide fermentation with starch as the carbon source at different levels of dissolved oxygen has been investigated. Experimental results showed that the dissolved oxygen has little influence on the fermentation of polysaccharide when its level exceeds 20% of air saturation. When the dissolved oxygen drops below 20% of air saturation, it has greater influence on the growth of bacteria. The optimum value of dissolved oxygen for bacterium growth is 25% of air saturation.