Abstract: Oxygen isotopic compositions of 29 Tertiary basalt samples from the Jiyang depressional basin are reported in this paper. The δ18O values of these basalts mostly range from 6.3‰ to 12.2‰. The Late Tertiary basalts with weak alteration haveδ18O values of 6.3‰0～7.9‰, which are similar to those of Late Tertiary basalts from central and eastern Shandong and are essentially in the range of normal continental basalts. However, the Early Tertiary basalts with evident alteration haveδ18O values of 6.7‰～12.2‰, more than 90% samples of which have δ18O values greater than 7.5‰. The obvious change ofδ18O values for Early Tertiary basalts not only occurs in the samples of different stratigraphic units, such as Shasan Member(Es3), Shasi Member (Es4)and Kongdian Formation(Ek), but also in the samples of the S31TIe stratigraphic unit collected from the same drill hole. Combined with the petrological and petrochemical data, it is clear that the increase ofδ18O values for the Early Tertiary basalts in different degrees was mainly caused by meteoric-hydrothermal alteration (carbonatization) at a lower temperature.