A recombinant PBR322 plasmid with inserts of operator sequence for lambda repressor and Bgl Ⅱ site was constructed. The interaction of lambda repressor to its operator within the recombinant plasmid was evaluated by fluorescence changes of intercalated ethidium bromide produced by Bgl Ⅱ digestion. The crude preperation of lambda repressor expressed in E. Coli showed a significant binding activity to its operator. According to the fluorescence changes, the consensus operator with an exact twofold symmatery sequence showed lower affinity to Rep than natural OR1.