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Seeking the optimal design with a given number of runs is a main problem in fractional factorial designs(FFDs). Resolution of a design is the most widely usage criterion, which is introduced by Box and Hunter(1961), used to be employed to regular FFDs. The resolution criterion is extended to non-regular FFG, called the generalized resolution criterion. This criterion is providing the idea of generalized minimum aberration criterion in non-regular designs. In this paper, we present these criteria and illustrate the advantages of non-regular designs in estimating the factorial effects with a smaller number of runs than that of regular FFDs. Some examples will be given.