HETP (Height Equivalent to a Theoretical Plate) is widely designated as the column efficiency in chromatographic separations. The effects of experimental variables such as concentration, injection volume, flow rate and composition of the mobile phase, column diameter, and column length on HETP were investigated by preparative liquid chromatography. Water and methanol as an organic modifier were used as the mobile phase. A sample of thymidine was injected into C₁₈ columns with different dimensions. from fifty experimental runs, it was shown that the larger amounts of sample and higher flow rates increased HETP and an optimum HETP existed at a column diameter of 7.8 m for preparative packings used in this experiment. The experimental values of HETP were correlated into a quadratic equation with the interaction terms based on the logarithmic experimental values of the experimental variables ; its regression coefficient was 0.952. The experimental variables were simulated from the correlation equation, and their effects on HETP were discussed.