Abstract: A criticality accident occurred on September 30,1999 at the uranium processing plant in Tokaimura Japan, which is the most severe accident since Chernobyl catastrophe. 213 people were exposed to radiation,among them 2 workers were exposed to 16~23Gy and 6~10Gy individually, one worker was 2Gy, 2 people was 10mSv and 208 person was 0~5mSv. I was invited to attend an international symposium on "The Criticality Accident in Tokaimura Medical Aspects of Radiation Emergency" in Chiba Japan on December 2000. An overview of the accident, dose estimation and neutron relative biological effects are discussed in this article.