Abstract: The internal standard method is used to determine trace 239 Pu in graphite. 233 U is used as internal standard for graphite powder test samples. Then plutonium in sample is leached in nitric acid, and then upper layer limpid liquid is removed to prepare for α source. The counting ratio of 233 U to 239 Pu is determined by an α spectrometry, and the quantity of 239 Pu is determined. When the quantity of sample is 125 g, the content of 239 Pu determined is 0.1～10 μg/g with a relative standard deviation is better than 2 %.