A process simulator for prediction of separation performance of a cryogenic air separation plant by packed columns with structured packing is developed by use of our heat and mass transfer model. The basic equations for the simulation are mass transfer correlations which were proposed in our previous work (Egoshi et al., 2000). Comparison of observed data from a pilot-scale cryogenic air separation plant, which consisted of a 710 mm-diameter high pressure sieve plate column for feed to low pressure column, a 500 mm-diameter low pressure packed column with structured packing for production of pure grade nitrogen and oxygen, and a 380 mm-diameter argon column with structured packing for production of crude argon is made. Predicted concentration profiles in the low pressure column and the ones in the argon column show good agreement with observed data. The effect of liquid feed location, side-cut location, vapor-liquid ratio of the feeds to the low pressure column on purity of the product nitrogen, oxygen and crude argon are also discussed.