Bacillus thruingiensis (Bt) strains are common bacteria in insect control field. Cadherin like protein receptor is consider as a key factor for binding Cry1A toxin to the Burch Border Membrane Vesicles (BBMV) in insect midgut. Our previous investigations have shown the importance of cadherin like protein as a synergistic factor to increase the toxicity of different types of specific Bt toxin against Lepidopterous insects.  Small fragment of cadherin receptor (CadF3) of BtCry1Ac toxin of Helicoverpa armigera (Húbner) pest was isolated, cloned and expressed. The recombinant bacterial strain of BtCry1Ac toxin is inactive to cotton leafworm; Spodoptera littoralis (Biosduval). In the current study, the ability of CadF3 fragment to enhance the toxicity of BtCry1Ac toxin against cotton leafworm; S. littoralis was examined. The results revealed that CadF3 has a potential effect to BtCry1Ac to become active toxin on cotton leafworm. Thus, this data showed the capability of CadF3 fragment to play very essential role to activate BtCry1Ac in cotton leafworm midgut.