Abstract: Objective A pilot study of ultrastructure of blood-brain barrier (BBB) after whole brain irradiation treatment in rats. Methods Twenty Sprague-Dawley rats were derided into 5 groups, To observe the changes of ultrastructure of BBB in 16 h after whole brain irradiation with ⁶⁰Coγ ray in rats,using lanthanum trace labeling with electron microscopy. Results 1.The lanthanum was only seen in the lumens of capillaries but not in the extravessels to the rats of unirradiation. 2.A few tight junctions of capillaries endothelial cell opened through which the lanthanum entering the outside of capillaries but not in the extravessels after irradiation 10Gy. 3.Most tight junctions of endothelial cell opened through which the lanthanum entering the extra cellular space but not enter into the neurons in 20Gy group. 4. The lanthanum through tight junctions of capillary vessel and basement membrane entered the plasma, mitochondria and endoplasmic reticulum of neurons after irradiation treatment 30Gy. 5.To the 40Gy group, Lanthanum nitrate particles existed in the brain tissue diffusely and a few endothelial cells and neurons lost normal configuration. Conclusions 1.The effect that radiation can directly damage the function and structure of BBB were enhanced proportionally with irradiation doses. 2.It is optimum time of chemotherapy after the whole brain irradiation in 30 gray. 3.The whole brain radiotherapy could not more than 40Gy, or it result in irreversible brain injure.