The effect of packing material length on mass transfer coefficient was studied for distillation experiments of binary system, acetone-ethylformate, in a 26 ㎜ I.D. laboratory column designed to simulate the flow characteristics of an industrial packed column. Overall mass transfer coefficients were obtained from the experiments on 4 different types of packings and an empirical correlation was proposed between overall coefficient and packing size. The analysis of the experimental result showed that the resistance in liquid phase was negligible and that the variation of individual mass transfer coefficients owing to different packing size could be well explained by an application of penetration theory to gas phase.