For the laser fluorination analysis of oxygen isotopes in silicate and oxide minerals, oxygen gas is commonly used as working gas and molecular sieves are used to absorb and transfer the oxygen gas into the inlet system of IRMS. In doing so, activation of molecular sieves has the influence on the efficiency of gas absorption/desorption and thus on oxygen isotope fractionation in vacuum extraction procedures. A series of experiments were carried out by means of 13X molecular sieves to test the influences. The results show that oxygen isotope fractionation could occur during adsorption and desorption of oxygen gas if the molecular sieves have different degrees of activation. At lower temperatures the molecular sieves are not able to be activated or only incomplete activation. As a result, the absorption capacity of molecular sieves is poor and measured δ18O values are fluctuated. Nevertheless, if the molecular sieves were heated at 200℃ meffle furnace for 24hrs and then timely transferred to the vacuum system and heated at 100℃ and in 10-3 Pa for 12hrs, their absorption is much enhanced relative to those untreated and slightly done at the lower temperatures. This leads to an external precision of ±0.05‰ (1σ) for oxygen isotope analysis. If molecular sieves were contaminated by BrF5, their absorption capacity is awfully reduced to cause measured δ18O values to be lower than the expected ones with the largest fractionation is up to 0.7‰. If molecular sieves were polluted by water vapor, there is reabsorption at the desorption temperature to cause measured δ18O values to increase with time. Hence,the molecular sieves have to be activated before the vacuum extraction and renewed timely when finding either pollution or the fluctuated δ18O values.