Abstract: It has been well-known that the optimum reflux ratio and the optimum packing size are importantcharacteristics of a packed column,especially for close-boiling fractionation.A correlation is proposed toestimate the NTU at finite reflux and from which a relationship to find the optimum reflux ratio is de-rived based on minimum cost.It is found that the maximum value of optimum reflux ratio is about 1.4R_M.For the study of optimum packing size,experimental work is undertaken with n-heptane-methylcyclohex-ane system in four laboratory columns ranging from 20 to 44 mm in diameter and being packed with 2.5×2.5mm angular helices.The optimum ratio of column diameter to packing size is found to be 13-14which is in rough agreement with that for ordinary or industrial size of packing.