In order to clarify the fundamental relation between fluid mixing and flow field, mixing processes in a time-periodic cellular convection are numerically analyzed. The effects of amplitude and frequency of the perturbation flow on mixing rate, mixedness and final mixing pattern are quantitatively analyzed with new measures of mixedness. The increase in amplitude of perturbation enhances both the mixing rate and the mixedness, but brings about no quantitative change in the mixing pattern. On the other hand, change in the frequency reconstructs the fundamental structure of the mixing pattern and causes serious and complex changes in the mixing rate and the final mixedness, because the frequency relates to the resonance condition of the flow field.