Abstract: The aim of this study is investigation of extraction ability of some lanthanide (III) ions; La3+, Ce3+, Pr3+, Eu3+ and Er3+, by using 18C-6, DB18C-6, DCH18C-6, DBP18C-6 as extractant in presence of trichloroacetic acid, from acidic solutions, in DMSO/water system. In this regard influence of trichloroacetic acid concentrations on stability and selectivity of extraction were studied. Experimental results showed that separation efficiency of mentioned ions with 18C6 and DCH18C6 in the presence of trichloroacetic acid, as counter anion, was improved and showed that is far more efficient than with DB18C6 and DBP18C6 or when nitric or acetic acid is used instead of trichloroacetic acid. Effective concentration of trichloroacetic acid was optimized and order of separation factors as 18C6 > DCH18C6 > DB18C6 > DBP18C6, were found for each lanthanide ions. Also, obtained distribution coefficient values of lanthanide ions decreased with increasing atomic number of ions. The experimental conditions are presented and chemically structure effect of derivative crowns on separation efficiency will be discussed. Determination of lanthanide (III) ions were performed with ion chromatography techniques.