Abstract: The effect of organic water-miscible solvents, such as methanol, ethanol, acetone, dioxane, glycol, dimethylsulfoxide(DMSO) ,dimethylformamide(DMF), tetrahydrofurance(THF) in aqueous phase on the extraction of uranyl sulphate by tri-n-octylamine (TOA) has been investigated. All data obtained showed that the addition of alcohols, ketones etc. into aqueous phase brings about an increase of distribution ratio of uranium, whereas the addition of DMSO, DMF etc. brings about a decrease . In the present study, the regularity and mechanism of the extraction with TOA are further studied and discussed from the measurements of some physical properties, such as dielectric constant, interface tension etc..