Abstract: In order to evaluate the control effect of 11.1% sustained release tablet (SRT) of imidacloprid on melon aphid (Aphis gossypii Glover) in watermelon and its application safety to watermelon and environment, the experiments were conducted in open field during growing season of watermelon (Citrullus lanatus) at Beijing in 2012-2013. The 11.1% sustained release tablets of imidacloprid (equivalent to 0.08 g imidacloprid per plant) were applied under the roots of watermelon seedlings while transplanting. The results indicated that the control effect of imidacloprid SRT on melon aphid basically covered the whole growing season (60 d) of watermelon; it was of 83.3% control effect level at 42 d after transplanting/application (DAA) in 2013. The accumulated release ratios of imidacloprid SRT were 15% and 32% at harvest (60 DAA) in 2012 and 2013. The diffusion range of SRT in soils was smaller than that of imidacloprid technical material. At 30 DAA, the residue of imidacloprid SRT in soils beyond 10 cm below application site and beyond 6 cm side of application site were all less than 0.2 mg/kg. Furthermore, the residue dynamic of imidacloprid SRT in the soils 2-4 cm away from the application site in vertical direction and 2-4 cm away in horizontal direction all showed a similar unimodal curve when plotted with the time, with highest values reaching 1.91 mg/kg and 0.10 mg/kg, respectively (all appeared at 30 DAA). At 120 DAA, the residue values of imidacloprid SRT in vertical and horizontal direction were only 0.17 mg/kg and 0.03 mg/kg. In the base of watermelon plants (0-30 cm in 2012, 1-3 internodes in 2013) treated by imidacloprid SRT and TC, the imidacloprid contents were detected at 14 DAA, and imidacloprid contents in the base of watermelon plants in the remaining period were not detected. Imidacloprid residues in whole watermelon, melon rind and melon pulp treated by imidacloprid SRT were all less than LOQ in 2012 and 2013, also lower than the maximum residue limit (MRL) of 0.2 mg/kg set by Codex Alimentarius Commission (CAC).