Abstract: A full-length cDNA clone encoding DREB was isolated from a cDNA library prepared under drought stress by the gene-specific primer amplification from Populus simonii. The cDNA was 671 bp with an open reading frame (543 bp) capable of encoding a protein of 181 amino acids. The DREB genomic sequences of 36 unrelated individuals were cloned and analyzed. A trinucleotide repeat motif of the CAC structural unit was detected within the coding region of DREB. This simple sequence repeat (SSR) was of 7 types, including 4(A), 5(B), 7(C), 8(D), 9(E), 10(F) and 11(G) times. To evaluate the diversity of the target SSR in the natural populations of P. simonii, SSR specific primer pairs were used to test the genetic structure in 16 natural populations of 528 individuals covering 11 provinces of China. The frequency distribution of the SSR allele from high to low was D＞C＞F＞E＞A＞G＞B in 16 populations. With mean effective number of alleles of 3.167 0, the mean value of observed heterozygosis and expected heterozygosis of the SSR allele was 0.468 5 and 0.531 5, respectively. Most populations with this SSR loci exhibited more or less inbreeding effects and deviated from Hardy-Weinberg equilibrium. Except for the populations of Ningwu (Shanxi province), Chaoyang (Liaoning province) and Yichuan (Henan province), this locus was neutral in all the other populations by Ewens-Watterson central testing. These results provide an important theoretical foundation for evaluating genetic diversity of germplasm resources with SSR markers within candidate genes and its application in conservation and use of breeding resources in forest trees.