Heat generated in electric double-layer capacitors (EDLCs) during charging or discharging includes irreversible and reversible heat, the former is caused by the Joule effect, the latter is attributed to the ion adsorption/desorption at the electrode/electrolyte interface. In this study, the reversible heat of EDLCs is discussed with the surface Gibbs free energy at the electric double-layer, with the model the calculated temperature behaviors for the EDLCs with liquid electrolyte are compared with some experimental data. Temperature profiles and oscillation amplitudes in galvanostatic cycling which are affected by reversible and irreversible heats are discussed in detail. The relations of reversible heat with current as well as electrolyte temperature and concentration are analyzed as well.