The effect of over-expression of AtDREB1A gene in chrysanthemums on heat stress tolerance was investigated. Transgenic plants with 35S:AtDREB1A construct (henceforth, 35S plants) and wild type (WT) plants were exposed to 45°C as a heat stress treatment. When heat-treated for 36 h and followed by a 3-week recovery, approximately 70% of the 35S plants, but less than 20% of the WT plants were survived. Leaf electrolyte leakage was significantly lower in 35S plants than in WT plants. cDNA macroarray analysis showed that 55 of 74 DREB1A regulon members were up-regulated under heat stress in WT plants. In comparison with WT plants, 35S plants displayed greatly enhanced expression of the genes including signal transduction, transcription and HSP70 in the early time, and the genes including photosynthesis and metabolism in the late time of the heat treatment. We also found that the 35S plants maintained significantly higher photosynthetic capacity, and elevated activity of Rubisco and sucrose-phosphate synthase under heat stress. These results suggest that improvement of heat stress tolerance in transgenic chrysanthemum may be associated with enhanced tolerance of photosynthesis.