The transition structures for the epoxidations of ethylene and the disproportionations by the dioxiranes of phosphines, phosphites and sulfides were studied with density function theory methods using the Becke3LYP functional and 6-311+G(2d,p) basis set. When the dioxiranes have methyl substituents rather than hydrogen substituents, the reaction barriers ($E_{TS}$) become higher in their epoxidations of ethylene by the steric hindrance, but become lower in their disproportionations of phosphines, phosphites and sulfides, which indicates that the nature of the dioxiranes seems to be electrophilic and in their disproportionations the reaction barriers are effected both by the electrophilicity and the steric hindrance. The steric factors in the disproportionations were calculated and more bulky substituents at dioxiranes may be necessary to retard the disproportionation and to enhance the epoxidation.