The present invention relates to a method for preparing 186/188Re-labeled human serum albumin (HSA) microspheres by 186/188Re(I)-tricarbonyl ion . This radioactive particle can be subjected to radioembolization for liver tumor. In this method, 186/188Re(I)-tricarbonyl ion (186/188Re(OH2)3(CO)3)+) are employed as a precursor for directly labeling HSA microspheres with 186/188Re at appropriate temperature.