The stability constant for the complex of $UO_2^{2+}$ with a macrocyclic aminoether ligand, 1,7,10,16-tetraoxa-4,13-diazacyclooctadecane, has determined in aqueous solution. The conductivity and pH metric measurements suggest that the ligand forms a stable 1:1 complex with $UO_2^{2+}$ ion, and the complex is an ionic form, $UO_2L^{2+}$, in aqueous solution. The fact that the ligand does not form a complex with lanthanides, such as $Ce^{3+}$, <TX>$Sm^{3+}$</TEX>, and $Nd^{3+}$ ions, in aqueous solution suggests a possibility of separation of the lanthanide elements from uranium matrix using the macrocyclic aminoether ligand.