Abstract: As to multicompoent distillation calculation, most available methods were proposed for operating problems. Although these methods can also be used for design problems, more computer time is needed, since the plate numbers are regarded as iterative variables. In this paper, the authors develop a new plate-to-plate method, which can directly be applied for design purpose and can be used for providing a rigorous solution. Ratios of molar flow rates and mole fractions are chosen as iterative variables. As to ideal systems and those close to ideal ones, these ratios refer to either ratios of molar flow rates of the heavy components or to those of the light components in various plates. As to extractive distillation processes, these ratios refer to the ratios of mole fractions of the components to be seperated in various plates. In comparison with the new plate-to-plate method for performance prediction[1,2] design calculation involves only week loops and more rapid convergence can be achieved without any acceleration or relexation, owing to the disappearance of the strong loop. As a plate-to-plate method, optimal location of feed plates and side-stream plates can easily be determined and no more optimization and any thumb criteria are needed.