An improved process of producing 1, 3-dialkyl-2-imidazolidinones directly from N, N'-dialkylethylenediamine and urea with a high yield is provided, which process comprises reacting a N, N'-dialkylethylenediamine of the formula ##STR1## wherein R represents --CH.sub.3, --C.sub.2 H.sub.5, --C.sub.3 H.sub.7 or --C.sub.4 H.sub.9, with urea at 180.degree. C. or higher in the presence of a polar solvent, to obtain a 1, 3-dialkyl-2-imidazolidinone of the formula ##STR2## and preferably comprises carrying out the reaction at two stages, that is, the initial period reaction being carried out at 140.degree. C. or lower to from a 1, 1'-dialkyl-1 1'-dimethylenebisurea and successively the latter reaction being carried out at 180.degree. C. or higher.