A new degassing model of 40Ar coupled with thermal evolution of the mantle is constructed to constrain the temporal variation of seafloor spreading rate. In this model, we take into account the effects of elemental partition and solubility during melt generation and bubble formation, and changes in both seafloor spreading rate and melt generation depth in the mantle. It is suggested that the seafloor spreading rate would have been almost the same as that of today over the history of the Earth in order to explain the present amount of 40Ar in the atmosphere. This result may also imply the mild degassing history of volatiles from the mantle. ¿ American Geophysical Union 1993