Abstract: Although routine sample mass (about 1 mgC) prepared by flam sealed tube Zn/Fe method can obtain the measurement data with high precision, accuracy and low background, most of sub-milligram (<1 mgC) sample preparation are based on online H₂/Fe method rather than flam sealed tube Zn/Fe method. In the present study, sub-milligram (between 0.04-1.0 mgC) samples with the modern radiocarbon standard of OXⅡ and the background sample of IHEG-Coal were prepared based on flam sealed tube Zn/Fe method. The effect of sample mass on beam current was discussed. The magnitude of exogenous modern-carbon and exogenous dead-carbon contamination on sub-milligram mass sample was calculated, subsequently evaluated the capability of the small sample preparation vacuum line. The results show that there is a significant positive correlation between the beam current and the sample mass. The exogenous modern-carbon contamination is estimated to be 1.0-3.0 μgC during the whole preparation procedure, and that is 0.5-3.0 μgC for dead-carbon. The precision and accuracy of ¹⁴C results are strongly depended on the sample size. When the sample mass is more than 0.5 mgC, the F_m value of standard sample OXⅡ is 1.342 7±0.003 9 and the age of background sample IHEG-Coal is (46 192±301) a, which shows that the precision and accuracy are reliable for this system.