The 20 kb DNAs are associated with crystals in many subspecies of Bacillus thuringiensis. We isolated 20 kb DNAs from crystals of B. thuringiensis strain 4.0718, then constructed a gene library using DNA fragments of Sau3AI partial digestion and pbluescriptIISK(+) vector. We screened out 440 recombinants, yielding a genomic coverage of ten and including 99% sequence of DNA which achieved the required theoretical value to construct the gene library. Through NCBI Blast and homology analysis, the sequencing results proved that the DNA came from the chromosome of B. thuringiensis. Moreover, we have completed the multiple alignment of homologous ropB protein sequences and phylogenetic analysis using bioinformatic software. For further investigation of the interactions between 20 kb DNAs and protoxins, molecular docking has also been done.