The CCT domain defines a conserved 43～45 amino acids of the 3' region of CONSTANS,CO-like,and TOC1.The plant specific CCT domain gene plays a role in light signaling and regulation of photoperiodic flowering.Meanwhile,the evolutionary relationship among family members has not been well studied.In this study,97 CCT domain containing genes were retrieved from four completely sequenced and annotated genomes.Analyses of corresponding protein structure and genetic relationships showed that CCT domain containing protein genes kept a wide range of variation within and among different species; Sizes of CCT domain gene family were increased through chromosome duplication.CCT domain gene family was classified into four subgroups according to the CCT domain.One sub-group gathered most of the grass CCT domain gene,which might be involved in the regulation of flowering time.The functional differentiation within and among species was comprehensively caused by the expansion of gene family members,structural variation of protein and patterns of gene expression.