Abstract: A core fragment highly homologous with the key gene of photoperiodic regulatory blossoming pathway, CONSTANS (CO) was obtained on the basis of preliminary Agapanthus praecox ssp. orientalis transcriptome sequencing results. The full length sequence of cDNA, the CO gene of A. praecox ssp. orientalis was gained by the RACE method, which was named as ApCOL with the GenBank accession number of KF683287. The results of sequence analysis showed that the full length of cDNA was 1 648 bp, which was composed of one open reading frame of 1 167 bp, 388 encoded amino acids, uncoded region (UTR) 5 and 3 with the length of 126 and 355 bp, respectively. The ApCOL had the typical domain of CO proteins: two B-box domains were boned at both of the hydroxy ends and a conserved domain CCT was located at the amino end. The results of homology comparison indicated that the ApCOL was highly homologous with the CO proteins of some plants such as Vitis vinifera (XP_002274384. 2), Theobroma cacao (EOX99181.1) and Glycine max (NP_001241023.1), with the homology of 50% or more. On the basis of phylogenetic analysis, it was demonstrated that ApCOL had the closest genetic relationship with clustering Triticum urartu ( EMS51329.1). The real-time quantitative PCR results suggested that the expression of ApCOL in leaves was obviously higher than that in blossom bud in the whole period of flower bud differentiation, and the highest and lowest expression of ApCOL in leaves and blossom bud were both happened in the period of flower bud induction; in the beginning-flower stage, the expression of ApCOL in the various organs from high to low was pedicel, leaf, young fruit, ovary, petal, stem and scape, the ApCOL expression in the pedicel was 2.68 and 114.3 times of that in leaf and stem; during different light cycle treatment, ApCOL gene expression patterns were similar, and were higly expressed during the dark treatment in A. praecox ssp. orientalis leaves. The results show that the ApCOL expression has obvious time and space difference and circadian regulation characteristics, and may play a crucial role in the processes of flowering and flower development.