Abstract: In order to relate the thermodynamic equilibrium of oxygen isotopes of the minerals and the validity of the Rb-Sr mineral isochron age for granitoids，oxygen，and Rb-Sr isotopic ratios of mineral and whole rock samples，as well as Ar-Ar ages of hornblendes and biotites were determined for two quartz monzonites from eastern Anhui province．The Tuncang and Hengshan bodies gave quartz (Q)-homblende (Hb1) oxygen isotopic temperature of 545±25℃ and 530±25℃ ，Q-biotite(Bt)temperature of 460±20℃ and 390±15℃ and Q-plagioclase(P1)temperature of 385±40℃ and 285±35℃ ．The relation of 0 isotopic temperatures suggested a retrograde equilibrium among minerals during cooling of the granitoids． Meanwhile，geologically meaningful Rb-Sr isochron ages were obtained ．The Tunchavg intrusion gave whole rock (w．r．)-Hbl-Pl Rb-Sr isochron age of 128．4±4．7 Ma．identical to the Hbl Ar-Ar plateau age of 125.51±0.55 Ma，and w．r．-P1-Hbl-Bt age of 120.3±2.6 M a，identical to the Bt At-At plateau age of 118.0±0.1 M a within uncertainty．For the Hengshan intrusion。the w．r．-Hbl-Pl Rb-Sr isoehron age of 108±15 M a is significantly younger than the Hbl Ar-Ar plateau age of 125.7±1.8 Ma, and the w ．r．-Hbl-Pl-Bt age of 118.6±1.2 Ma younger than the Bt Ar-Ar plateau age of 120.0±0.2 Ma．The young Rb-Sr isoehron ages with large uncertainties are due to alteration of the feldspar and small variation in Rb/Sr ratios in the rock．Therefore．we conclude that the equilibrium state of the oxygen isotope systems is a prerequisite for the geologically meaningful Rb-Sr isochron ages of the granitoids ．However，detailed mineralogical investigation is needed for obtaining a good isoehron．The cooling rate of about 10～20℃ ／Ma obtained by cooling curve built using oxygen isotope temperatures is 10～15℃ ／Ma lower than that of 20～35℃ ／Ma estimated by blocking temperatures，the reason for it merits further investigation