Abstract: Using two Brazilian nitrogen fixing sugarcane varieties(B1 and B8) were employed as the test materials,a barrel gravel culture was conducted to study the effects of molybdenum on nitrogenase activity of nitrogen-fixing bacteria of sugarcane under greenhouse condition.Four Mo concentrations were adapted: 0 mg/L,0.01 mg/L,0.02 mg/L,and 0.04 mg/L.The experimental results showed that, under nitrogenous conditions,low Mo promoted nitrogenase activity of nitrogenfixing bacteria in roots,while high Mo increased that in stems of both two sugarcane varieties,and leaves of the variety B1.Under non-nitrogenous conditions, however,all Mo treatments increased the activities of nitrogenase of nitrogen fixing bacteria in leaves and stems of B1,but decreased that in leaves,stems and roots of B8 as well as that in roots of B1 compared with the control.There was a negative correlation between the nitrogenase activities of nitrogen fixing bacteria in root and that in leaf and stem.All the results indicated that Brazilian sugarcane also had a nitrogen-fixing capacity in Guangxi,Mo was propitious to regulate nitrogenase activity of nitrogen-fixing bacteria in sugarcane under nitrogenous conditions,and promoted nitrogen fixing;while under non nitrogenous conditions,it was unpropitious to regulate the nitrogen activity and inhibited nitrogen fixation of nitrogen-fixing bacteria.