Abstract: The judgment on passivity and activity for a time-varying energy-storing network element is the most complicated problem. A set of criterions has been derived in this paper, based on the truth that the electro-magnetic energy, stored in a time-varying energy-storing network element, may be reversibly exchanged with the work done by the mecha nical force, which causes the nature of element characteristic to be changed. With these criterions, it is convenient to find out the passivity and activity for time-varying linear or nonlinear capacitor inductora and mem-ristor with two or more teminals.