Abstract: We screened 5′ end sequence of S-adenosylmethionine decarboxylase gene when identified 5′ end sequence of phytochrome C gene from tall fescue. The primers were designed and amplified 3′ end sequence based on the 5′ end sequence of S-adenosylmethionine decarboxylase gene, 5′ end sequence and 3′ end sequence were jointed, a SAMDC of cDNA from tall fescue has been identified, and designated as FaSAMDC(GenBank Accsssion: HQ606139). Theoretical basis of different photoperiods regulated FaSAMDC expression was provided through a study of FaSAMDC differentially expressed during 24 hours in long-days and short-day using real-time fluorescence quantitative PCR. The cDNA of FaSAMDC is 1 964 bp in length and contains three open reading frames, tiny upstream ORF (226-234 bp), small upstream ORF (234-380 bp), main ORF (555-1 742 bp) respectively. Main ORF encodes a protein of 395 amino acids. Domain analysis reveals that proteins encoded by FaSAMDC genes contain two conserved domains: Proenzyme splice site domains and involving SAMDC protein rapid degradation PEST domain. FaSAMDC showed a close genetic relationship with the known SAMDC of monocotyledonous plants. Three expression peaks was detected in both long days and short days condition. The level of FaSAMDC expression peak under long days was significantly higher than that under short days. However, the peak of FaSAMDC transcription level under short days condition was earlier several hours than that of long days condition. This indicates that FaSAMDC gene transcripted with a circadian rhythm and was regulated by photoperiod.