Abstract: The toxicity and efficacy of thiamethoxam to tobacco whitefly, Bemisia tabaci were examined through laboratory bioassay and field trial. Results showed that thiamethoxam had good activity to the 1st and 2nd instar nymphs, and the adjusted mortalities were over 90% at the highest concentration of 125 mg/L. The efficacy of leaf-dipping method and wrapping the leafstalk with thiamethoxam (treated cotton) were better than that treating upside of the leaf only. Meanwhile high effectiveness of thiamethoxam was also observed to adult of the whitefly. Efficacy of the systematic action was the highest,the next was leaf-dipping method, and the last was the method treating upside of the leaf only. In the field trial, drench application and spraying had good effect, the control efficiency reached about 95% after 1 and 3 days treatment, and the efficacy could sustain for over 14 days with drench application. These results indicated that thiamethoxam has excellent systematic efficacy and contact activity, strong penetrating effect and long-lasting residual activity, which provide farmers selectable application methods to obtain best control results.