The photosynthetic parameters of mung bean seedling in present of increased UV-B radiation,paclobutrazo(PP₃₃₃)or their combined treatment were determined with TPS-1 photosynthesis system or calculated according to the theory of Farquhar and Sharkey. The content of RuBP carboxylase-oxygenase (Rubisco)was also assayed simultaneously. In the first leaf of seedling,UV-B radiation resulted in decreases in net photosynthetic rate(Pn),stomatal limitation value(Ls)and stomatal conductance(Gs),which were accompanied by increase in intercellular CO₂ concentration (Ci). PP₃₃₃ elevated Pn,Gs and Ci,and reduced Ls. In comparison with PP₃₃₃ treatment,UV-B+PP₃₃₃ induced reduction of Pn,Gs and Ci,but promoted Ls to rise. Pn,mesophyl photosynthetic capacity(Ao),apparent quantum yield(AQY),carboxylation velocity of Rubisco(dPn/dCi) and the content of Rubisco were reduced both in UV-B treatment(in comparison to control)and UV-B +PP₃₃₃ treatment(in comparison to PP₃₃₃ treatment)seedlings,and the reductive extent was greater in UV-B treatment seedling than in UV-B+PP₃₃₃ treatment seedling. And the reductive effects of UV-B radiation on Pn,Ao,AQY,dPn/dCi and Rubisco content were bigger than the promotive effects of PP₃₃₃ treatment on those parameters. These results prove that UV-B radiation depresses Pn mainly by inhibiting photosynthetic activities of mesophyll cell,while the dominant reasons of PP₃₃₃ promoting Pn and UV-B radiation inhibiting Pn in UV-B+PP₃₃₃ treatment seedling are the changes of Gs.