The impact of UV-B radiation (280 to 320 nm) at the earth's surface is predicted to increase because of the anthropogenic depletion of stratospheric ozone caused by industrial emissions of atmospheric pollutants. Plant growth and productivity are compromised by excessive UV-B because it damages DNA, RNA, and proteins. Plants are thought to employ a variety of UV-B-protective mechanisms, including accumulation of a range of secondary metabolites, which in turn affect numerous physiological functions. Sinapate esters are a group of hydroxycinnamic acid derivatives found abundant in cruciferous plants, e.g., Arabidopsis thaliana. One of the functions of sinapate esters is to protect plants from harmful UV irradiation. It has been suggested that sinapate esters provide greater UV-B attenuation than flavonoids. Unlike flavonoids, studies of sinapate esters UV-B absorption are rare, despite their widespread occurrence throughout the plant kingdom. In the past decades, gene-knockout mutants have provided an opportunity to study the role of sinapate esters in protecting the leaves against the deleterious effects of UV-B radiation. However, none of these studies offered insights into the profiles of the sinapate esters in the wild-type plants in response to UV-B irradiation. Here we report the profiles of sinapate esters and the expression of related genes in response to UV-B radiation(40 μW/cm²)for 7 days in model plant A. thaliana. The seedlings were exposed to UV-B radiation at two different growth stages: young plants (1 week after planting, 2 cotyledons) and mature plants (2 week after planting, 4 rosette leaves). Chlorophyll a/b ratio, an indicator of plant damage caused by UV-B, was not significantly affected by UV-B radiation both in young and mature plants. It suggests that the intensity of UV-B used in our research is not harmful to A. thaliana. Results may also indicate under this UV-B radiation, plants have the ability for self-repair. Two sinapate esters were detected in A. thaliana leaves including sinapoylglucose (SG) and sinapoylmalate (SM). The sinapate esters were accumulated in response to 7 day UV-B exposure. The levels of sinapoylglucose and sinapoylmalate in young and mature plants were higher than those in control plants after UV-B treatment. The content of sinapoylmalate was an order of magnitude higher than sinapoylglucose in both young and mature A. thaliana. The proportion of sinapoylmalate increased with UV-B treatment, while the content of sinapoylglucose showed a significant decrease. The significantly higher expression of SNG1 was also observed in UV-B treated plants than in control. These results indicate that sinapoylmalate biosynthesis plays an important role in response to UV-B radiation. In addition, the contents of two sinapate esters in young plants were more than ten times higher than those in mature plants since the biosynthesis related genes, FAH1 and SNG1, showed higher expression in the young plants. Meanwhile, the amplitude of sinapoylglucose and sinapoylmalate variations in response to UV-B is substantially greater in mature plants (21.88% and 70.63% respectively) than in young plants (7.01% and 6.05% respectively). These results indicate that sinapate esters are effective UV-B protectants, which exhibit constitutive defense response in young plants and inducible defense response in mature plants.