20ɑ-hydroxysteroid dehydrogenase (20ɑ-HSD) enzyme converts progesterone into biological inactive steroid, thus playing a key role in the termination of pregnancy or estrus cycle and allowing parturition and ovulation to occur in most mammalian animals. However, function and regulation of this enzyme has not known well in primate reproductive physiology. We previously demonstrated the expression level and localization of the 20α-HSD in the reproductive tissues of macaque monkeys of pre-ovulation and pre-parturition period. Also, we amplified about 2005 bp 5'-flanking region from placenta genomic DNA and examined methylation pattern and promoter activity. In present study, we focus on the analysis of molecular characterization of the promoter region by using reporter assay systems. We constructed of deleted mutants (— 890 bp; HSF-2), (— 513 bp; XFD), (— 276 bp; Ap-1) and (— 72 bp; Sp-1) and each mutants were cloned into pGL3-basic vector. These deletion mutants were transfected into CHO cells and co-transfected with Sp-1 or Ap-1 transcription factor plasmids. Compared to — 890 bp and 513 bp promoter fragments alone, transcription activity increased when these constructs were co-transfected with Sp-1 and Ap-1 factor. However, for the absence Ap-1 factor binding site in 276 bp fragment activity dramatically decreased in both transfections. Next, we constructed of 306 bp fragment which is including of Ap-1 binding site and nucleotides converted mutants of the Ap-1 factor binding site. In this result, 306 bp fragment's transcription activity was high as wild type. However, the mutant activity which converted Ap-1 site’s all nucleotide was significantly decreased. These findings are confirmed by gel-shift assay examining Ap-1 binding site on the 20 α-HSD gene upstream region and expression of Ap-1 factor was determined by RT-PCR and Western blot in pre-parturition period placenta and CHO-K1 cell line. Our results indicate that Ap-1 site (— 281 → — 274) (5'-TGTCTCAT-3') plays a crucial role for monkey 20 α- HSD gene transcription.