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A new technique for the removal of isobaric interferences on Ba has been developed for ICP-mass spectrometer. Mass spectrometric interferences on Ba by 138La+, 136Ce+, or 138Ce+ isobaric signals were effectively eliminated by a selective production of oxide ions. The La+ and Ce+ ions could be quantitatively converted to oxide ions (LaO+ and CeO+) by the ICP ion source under the low-power operation. Production efficiency of oxide signals for La and Ce was remarkably enhanced, i.e., >2,000,000 times the level obtained by the conventional ICP power (1.35 kW). Residual signal intensity of La+ signal was almost the same level as the instrumental background. The overall contribution of isobaric 138La and 138Ce to 138Ba signals were 0.02% and 0.1%, respectively, even with the presence of × 10,000 times higher concentration of La and Ce. The data obtained here demonstrate that the level of the isobaric interferences by 138La, 136Ce, and 138Ce signals produced by the low-power ionisation was negligible within the analytical precision achieved in this study (∼1%). It should be noted that the analytical sensitivity for Ba obtained by the low-power ICP remains almost 85% of the level achieved by the conventional ICP power. This is very promising because the present technique can be applied to the laser ablation sampling technique which requires high sensitivity without chemical separation or purification procedures.