The minimal relative retention α’ and the required relative retention α’_(min) of the pair of substances being most difficult to separate in a multicomponent mixture have been discussed on the basis of the fundametal retention equation ln k’=b_0+b_1C_B+b_2ln C_B and resolution criterion. The selection principle to optimize elution composition means that selected a value must be greater than α’_(min) and that the capacity factor of last peak should be held to a minimum. A rapid method to select optimum elution composition has been established in which it is necessary to do a few experiments. An example for the 5 pairs of phenylthiohydantion amino acids (PTH-AA) using the ternary mobile phase of methanol, ether and hexane is also demonstrated in this paper. The results show that theoretical prediction is good in agreement with the experiments. The minimal relative retention α ’and the required relative retention α’ _ (min) of the pair of substances being most difficult to separate in a multicomponent mixture have been discussed on the basis of the fundametal retention equation ln k ’= b_0 + b_1C_B + b_2ln C_B and resolution criterion. The selection principle to optimize elution composition means that selected a value must be greater than α ’_ (min) and that the capacity factor of last peak should be held to a minimum. A rapid method to select optimum elution composition has been established in which it is necessary to do a few experiments. An example for the 5 pairs of phenylthiohydantion amino acids (PTH-AA) using the ternary mobile phase of methanol, ether and hexane is also demonstrated in this paper. show that theoretical prediction is good in agreement with the experiments.