We isolated 13 DREB1 (dehydration responsive element binding factor 1) genes from chrysanthemum and further divided them into three groups, DgDREB1A, DgDREB1B and DgDREB1C, based on the phylogenetic analysis. Each group showed their unique expression patterns under cold, dehydration and salt stress conditions. Arabidopsis plants overexpressing DgDREB1A (1A plants) exhibited significantly stronger tolerance to freezing and drought than those overexpressing DgDREB1B (1B plants) and the control plants. In addition, 1A plants showed delayed flowering, but not dwarfism; while 1B plants showed dwarfism, but not delayed flowering. In 1A plants, the expression of three stress-related DREB1-downstream genes, COR47, COR15A, and RD29A, was strongly induced while the expression of CO and FT, two photoperiod responsive flowering-time genes, was inhibited. In 1B plants, the expression of GA2ox7, a GA-deactivation enzyme gene, was dramatically enhanced. The results above strongly suggest that members from different DgDREB1 groups may have distinct effects on plant development: DgDREB1A may be involved in photoperiod-related flowering-time determination and DgDREB1B in GA-mediated plant development.