Previous paleomagnetic studies have revealed that pre-Quaternary pelagic clay with neither siliceous nor calcareous fossils has unstable remnant magnetization, and that alternating field (AF) demagnetization is not effective to remove the secondary magnetization. This study applied thermal demagnetization of pelagic clay from the South Pacific, and recovered characteristic directions. The observed paleolatitudes agree with those from plate tectonic analysis, which proves that the thermal degmagnetization worked well. The magnetic overprint is estimated to be viscous remanent magnetization (VRM) from VRM acquisition experiment. A hypothesis is proposed that the magnitude of the VRM of pelagic clay has been controlled by the grain size of magnetic minerals of eolian origin, which has been subjected to the intensity of the global atmospheric circulation. This can explain the Late-Pliocene unstable-to-stable transition of the remanence, which has been observed widely in the Pacific.