The epithermal neutron activation analysis （ENAA） using BN and BN+Cd as thermal neutron shielding materials was used to determine the contents of iodine in 20 Chinese biological standard reference materials （SRMs）. The analytical precision

expressed by the relative standard deviation

is 2%-6% and 8%-12% for solid samples with iodine level of more than 100ng/g and 20-100ng/g

respectively

and 12%-18% for liquid samples with iodine level of less than 100ng/g. The detection limits of iodine in our experimental condition vary between 10-30ng/g

55-95ng/g and 25-68ng/g for BN-shielding in inner irradiation site

Cd-shielding and BN+Cd shielding in outer irradiation site

respectively. Furthermore

four international biological SRMs were determined

and the experimental results agreed well with the certified or literature values.