Cry3 toxins from Bacillus thuringiensis are used as biopesticides and the transgenic crops to control of leaf-feeding beetles. Cadherin in insect midgut epithelium is identified as receptor for Cry toxins in several insects and some domains of it synergizes Cry toxicity. Cadherin (DvCad1-CR8-10) fragment of Diabrotica virgifera virgifera enhanced Cry3Bb toxicity to Colorado potato beetle (CPB), Leptinotarsa decemlineata. Single cadherin repeat (CR) fragment of DvCad1-CR8-10, have a strong binding affinity to the active Cry3Bb toxin. The dissociation constant Kd value of CR8, CR9, and CR10 were 4.9 nM, 28.2 nM, and 4.6 nM, respectively. Interestingly, CR8 and CR10 enhanced Cry3Bb toxicity against CPB and Lesser mealworm (LMW), Alphitobius diaperinus, neonates in up to 2-folds. The DvCad1-CR10 peptide is further analyzed by in-frame deletion to determine the active site for Cry3Bb toxin. The active site is narrowed down to a 26 amino acid locating in the N-terminal region of DvCad1-CR10 that either synergized Cry3Bb toxicity on the CPB and LMW neonates in 3-folds or bound to the toxin with high affinity. The extent of Cry3Bb toxin enhancement by the activie site in DvCad1-CR10 may have practical application for control of CPB and LMW.