This study was outlined to declare the effect of releases of Coccinella undecimpunctata (larvae and adults) and Chrysoperla carnea (larvae) at different predator: prey ratios (P: p) to control the cotton aphid, Aphis gossypii, on tomato plants under field cage conditions during summer season of 2003. Effective control of A. gossypii was gained after one day from introducing the larvae of the coccinellid predator when the P: p ratio was 1:15 or 1:30, meanwhile at higher ratios (1:60 and 1:75), the aphid numbers decreased after seven days from the release. The cotton aphid numbers decreased by 96.7 and 77.5% at 1:15 and 1:30 P: p ratios, respectively when adults of the coccinellid released. The number of aphids remained zero for a period of 13 days after the release. At the predator: prey ratio of 1:45, the reduction percentage of aphid densities was 47.8 and 86.1% after one and four days. Whereas, at the higher ratios (1:60 and 1:75), the reduction percentage was 37.1 and 33%, 77.1 and 62%, and 92.1 and 88% after one, four, and seven days from the release, respectively.Chrysoperla carnea larvae at a P: p ratio of 1:15 yielded excellent control of A. gosspii with the reduction percentage 88.3% after one day from the release. There was a drastic drop in the aphid populations after chrysopid larvae release at the P: p ratios of 1:30 and 1:45. The reduction percentages were (64.2 and 39.4%), and (96.7 and 81.7 %), respectively after one and four days from the release.According to regression analysis between P: p ratios of C. undecimpunctata (larvae and adults) and larvae of C. carnea and reduction percentage of the cotton aphid, there were negatively high relationship of both predators, which means that the reduction rate was increased with lower P:p ratios and vice versa.