Diels-Alder reaction of cyclopentadiene and butadiene gives several dimers, which are considered to be valuable materials for various organic preparations. The cis-dihydroxylation of these dimers with N-methylmorpholine-N-oxide in the presence of a catalytic amount of osmium tetroxide was investigated to examine the regio- and stereoselectivities of the reaction. The hydroxylation of 4-vinyl-1-cyclohexene occurred exclusively at the double bond of cyclohexene ring to give two stereoisomeric mixture of cis-diols 2a and 2b. 5-Vinyl-2-norbornene led to the cis-diol 4 which was hydroxylated regio- and stereoselectively. The same reaction of cis-3a, 4, 7, 7a-tetrahydroindene gave cis-diols, 6 and 7, and tetrol 8 with a high stereoselectivity. On the other hand, the cis-hydroxylation of endo-dicyclopentadiene gave diols, 10 and 11, but that of exo-isomer afforded only cis-diol 13. Cis-dihydroxylation of norbornene ring and that of cyclopentene ring proceeded stereoselectively by one side attack, but that of cyclohexene ring occurred at both side of double bonds. It was assumed that these selectivities depended on the reactivity of double bond and on the steric effect between the substrate and oxidizing reagent in the transition state.