Basaltic rocks from twenly-three lava flows of late Pliocene in New Mexico and Arizona, U. S. A., were studied by means of Thellier's method and seventeen intensity estimates were obtained. It was observed for both originally normal and reversed samples that the field intensity at the time of lave-flowing was not greatly different from that of the present geomagnetic field at the sampling locality. Seventeen estimates of the intensity of the geomagnetic field in the late Pliocene range from 0.35oe to 0.82oe, the mean valve being 0.53±0.14oe. It has been confirmed that the direct comparison of the magnitude of natural remanent magnetization with that of total thermoremanent magnetization produced in laboratory could not give reliable intensity values in many cases, because most samples show some changes in magnetic properties during the heating process. According to the results obtained from Rio Grande lavas which cover about one million years in the late Pliocene, the intensity fluctuations of the geomagnetic field seem to have a much shorter period than the duration time of a geomagnetic polarity epoch.