We discuss the capacitance of a coherent mesoscopic parallel-plate capacitor based on the self-consistent theory of dynamic response for mesoscopic systems. The results show that the capacitance is a complex function of frequency which is strongly dependent on the frequency of the external field. The capacitance-frequency curve shows a significant characteristic that a peak in the imaginary part of the capacitance corresponds to the minimum in the real part, and further study shows that they are related to a plasmon-like excitation. In addition,we discuss the size effect of the capacitor, and find that the capacitance approaches geometric capacitance when the distance between two plates of the capacitor is very large.