Sulfur is an essential element for plant growth. After absorbed from soil by plant roots, sulfate is firstly activated by ATP sulfurylase to form adenosine 5'-phosphosulfate (APS). There are primary and secondary pathways for further conversion of APS, namely reduction of APS by APS reductase to form sulfite, and the phosphorylation of APS by APS kinase (APSK) to form 3'-phosphoadenosine-5'-phosphosulfate (PAPS) as the sulfate donor for the sulfation reactions. Recent researches demonstrated that APSK from Arabidopsis thaliana has an important role in regulating the primary and secondary metabolism of sulfate, and that the formation of intersubunit disulfide bonds between C86 and C119 under oxidation state would decrease its catalytic efficiency dramatically. It had not been reported that whether the rice isozymes were affected by redox environments or not. Sequence analysis demonstrated that two rice APSK (OsAPSK) sequences contain corresponding cysteines homology to Arabidopsis thaliana involved in disulfide formation. In this study, OsAPSK1 and its C36A/C69A mutant were prokaryotically overexpressed, purified and assayed for activity of APS phosphorylation. Activity depression for OsAPSK1 under oxidative stress and no for its mutant C36A/C69A suggested that OsAPSK1 was regulated as proposed previously, and that depression of APSK activity in the secondary pathway would facilitate the primary pathway for sulfate reduction to enhance the cell redox buffering capacity. Further studies such as relations between PAPS contents and oxidative stress, and regulation of different APSK isoenzymes are being investigated.