A favorable isotherm open to the right to some extent is better than a linear isotherm to give shorter column height in pressure swing adsorption(PSA). We face to a problem how favorable isotherm is actually favorable in PSA operation. The purpose of this paper is to propose how to construct such a custom made isotherm which optimizes the PSA processes. The PSA model was much simplified to approach this complicated problem.The first simplification is a method called the short cycle time approximation, in which the time-dependent partial differential equations were simplified to steady state equations for a short half cycle time. As the next simplification, the HETP(height equivalent to a theoretical plate) concept was applied to predict the total column height of PSA. The total height was found to be the sum of the reciprocal of average gradient of adsorption isotherm weighted with a ratio of gas velocity in desorption step to that in adsorption step. By use of this result, the most “favorable” isotherm to give the shortest column height was constructed by increasing the gradient of isotherm in the next stage by a factor of square root of the velocity ratio. Thus obtained isotherm was better to shorten the column height than any available isotherms even with the best parameters.