Palindrome sequence is very common in DNA, it usually acts as cis-elements of many regulative factors. In this paper, we design two palindrome sequences which is different in length, construct Cre-loxp combination system, then co-transform two kinds of plasmids, one contains cre gene, the other contains loxp site to E. coli DH5a strain. We judge the expression degree of cre gene in E. coli by examining the changes of plasmid maps of co-transformed strains, then conclude that 20 bp or shorter than 20 bp palindrome sequence uninhibit the expression of downstream genes, however, 34 bp or longer than 34 bp palindrome sequence can inhibit that.