Cyclobutane pyrimidine dimers and monomers of thymine were separated on a C₁₈ reversed-phase high performance liquid chromatographic column. Using the two mathematical models, the effect of the sample size on peak shapes in preparative liquid chromatography was investigated. One of them is the linear kinetic model, and the other is based on the nonlinear adsorption isotherm. In case of small injections of sample, the good agreements between the calculated value by the linear kinetic model and experimental data were achieved. However, when sample size is increased, this model lacks in the prediction of large concentration profile of sample. However, the nonlinear model permits the accurate prediction of the location of the component band and the determination of the appropriate time to start and stop collection of the enriched fraction at the higher concentrations of monomer and the lower that of dimer. Therefore, extremely small amounts of dimer can be extracted from monomer solutions.