Ultraviolet-B (UV-B, 280-320 nm) constitutes a minor part of the solar spectrum, which can be absorbed by stratospheric ozone layer. However, a global depletion of the ozone layer, largely due to the release of man-made chlorofluorocarbons, has resulted in an increase of ground-level solar UV-B radiation. UV-B can influence plant processes, either through direct damage or via various regulatory effects. Many researches have warned that excessive UV-B radiation can harm living organisms by damaging DNA, proteins, lipids, and membranes and consequently affecting plant growth, development, morphology, and productivity. Two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) is a powerful technique for resolving hundreds of proteins in parallel. Combined with mass spectrometry (MS), it allows rapid and reliable protein identification. In recent years, proteomic-based technologies have been successfully applied to systematic studies of the stress responses many plant species, including Arabidopsis, soybean, rice, wheat, barley, potato, tomato, and many others. A wide range of abiotic stresses have been examined, such as drought, nutrition deficiency, temperature, oxidative stress, herbicides, wounding, anoxia, salt, and heavy metals. These investigations have provided a wealth of important information on the physiological processes involved in plant stress responses. To explore the molecular mechanisms of the decreased photosynthetic rate and the resistance of peanut (Arachis hypogaea) when exposed to enhanced UV-B radiation, 2-D PAGE and MS were used to identify the differentially-expressed proteins in peanut seedling leaves in response to supplementary UV-B radiation (54 μW/cm²) for 24 h. A total of 39 protein spots were differentially expressed by at least 2.5 fold compared with the controls (22 proteins were down-regulated and 17 were up-regulated) after treatment with supplementary UV-B radiation. Of those protein spots, 27 were successfully identified by MALDI TOF/TOF MS after a database search. Those 27 proteins could be classified into eight categories according to their functions: class Ⅰ, photosynthesis (plastocyanin, ribulose-1,5-bisphosphate carboxylase small subunit, oxygen-evolving enhancer protein 1, PsbP domain-containing protein 6, and fructose-bisphosphate aldolase); class Ⅱ, carbohydrate metabolism (malate dehydrogenase); class Ⅲ, energy synthesis (ATP synthase); class Ⅳ, amino acid biosynthesis (cysteine synthase); class Ⅴ, protein biosynthesis (ribosome recycling factor); class Ⅵ, protein processing (heat shock proteins); class Ⅶ, defense responses (chitinase, peroxidase, Cu-Zn SOD, caffeic acid 3-O-methyltransferase, and germin-like protein); class Ⅷ, unknown proteins. In conclusion, we hypothesized that the enhanced UV-B radiation caused a decrease in the photosynthesis rate of peanut leaves mainly via three mechanisms. First, enhanced UV-B may down-regulate the expression of ribosome recycling factor, which caused a decrease in the expression of subunit PsbP in photosystem Ⅱ, thus destroying the thylakoid membrane structure. Second, the reduced plastocyanin expression may have induced a decrease in photosynthetic electron transport efficiency. Third, the down-regulation of ribulose-1,5-bisphosphate carboxylase and fructose-1,6-bisphosphate aldolase resulted in a decrease in carbon assimilation. At the same time, peanut may also enhance its resistance to UV-B stress by increasing the expressions of antioxidant enzymes and non-enzymatic antioxidants, germin-like proteins, pathogenesis-related proteins, and heat shock proteins. These results provide important information for understanding the molecular mechanisms by which A. hypogaea responds to elevated UV-B stress.