Abstract: To study the effects of paclobutrazol on growth,development,photosynthetic characteristics and chloroplast ultrastructure in Dahlia pinnata,the cultivar ‘Longshangxiongying’ was treated with different concentrations of paclobutrazol and water was used as the control. Plant height was dwarfed,internodes were shortened,flower diameter increased and ornamental values enhanced under different paclobutrazol treatments. Chlorophyll contents and net photosynthetic rate initially increased but then decreased and were highest at the 24^th day. Variation trends of transpiration rate,stomatal conductance and intercellular CO₂ concentration differed with increasing treatment time. The photosynthetic capacity reduction was due to both stomatal factors and non-stomatal factors. At concentrations of 75 mg/L and less,the chloroplast and mitochondrion structure were completed,the grana and grana lamella arranged appropriately,and the numbers of grana and grana lamella were higher than in the control,while the photosynthetic capacities were stronger. In the 100 and 125 mg/L treatments,chloroplast and mitochondrial structures were damaged and plasmolysis occurred. The growth of grana and grana lamella was incomplete. These factors seriously influenced photosynthesis. The effect of 75 mg/L paclobutrazol treatment was the best. In this treatment,chlorophyll contents of D. pinnata leaves were the highest,photosynthesis capacity was the strongest and the ornamental value of the D. pinnata were the highest.