Biological control of Aphis gossypii by natural enemies was carried out in watermelon from April to June. Aphidius colemani was released 3 times at rate of 2/㎡ to control A. gossypii(April 24, May 8, and May 22 in 2007). In A. colemani-released plots, A. gossypii showed low population density of < 0.6 per leaf with the highest parasitism of 57.2%, and 2.0~10.6% in the percentage of leaves with aphids. In control, the population of A. gossypii kept on increasing from early May and reached 653.2/leaf on may 29.