The proper measure of closed-loop performance variation in the presence of model-plant mismatch is discussed in this paper. A generalized closed-loop error transfer function, which is a special representation of the dual Youla parameter and has a close relationship with the pointwise v-gap metric, is proposed as the suitable means of representing closed-loop performance variation in case of plant perturbation, and the closed-loop performance variation measure is accordingly defined as its maximum singular value frequency by frequency. It is shown that this measure is essential and informative in characterizing closed-loop performance variation, This measure is also shown to be readily applicable to on-line closed-loop performance assessment or monitoring, even without the explicit model of the plant. Its variant, defined as the eta -function, which features the relative performance variation as well as generalized stability margin variation with respect to the nominal plant, is also discussed.