The unicellular green alga Haematococcus pluvialis accumulates a highly valuable ketocarotenoid, i.e. astaxanthin up to 4% dry weight under stress conditions. Phytoene synthase is considered to be the first rate limiting enzyme in carotenoid biosynthesis pathway in H. pluvialis. The cDNA and genomic genes of phytoene synthase, i.e. psy from H.pluvialis were cloned and characterized. Result showed that psy had one open reading frame of 1 200 bp encoding a putative polypeptide of 400 amino acids which was interrupted by four introns. Phylogenetic analysis revealed that psy from green algae formed a monophyletic clade, and its closer relationship was higher plants. By using genomic walking approach, an approximate 1 kb 5′ flanking region of psy gene was cloned and a number of putative cis-regulatory elements were revealed. Fusing a 297 bp internal sequence (－297 to －1 bp from the translation initiation codon of psy) with the reporter gene, i.e. lacZ before attempted introducing the construct into the green alga via particle bombardment resulted in lacZ transient expression.