The present study describes systematically the ultrastructural changes of blood and some organs of rabbits, goats and rats with severe decompression sickness. In our experiment, the animals were exposed to pressure of 0.6 MPa and 1.0 MPa for 60 minutes respectively, to induce severe decompression sickness. The results are as follows: 1. Abnormality of platelets. The granules of platelets decreased and the platelet pseudopods were formed with adhesion and aggregation of platelets. 2. Deformation of erythrocytes. Triangle-like or shell-like cells, dacryocytes, twisted cells, and Ⅰ—Ⅲ type echinocytes were found. 3. Changes of endotheliocytes of the vascular wall became flattened. The space between these cells significantly widened (up to 4-18?m), and blood cells could be found to have wedged into it. 4. Formation of air-blood surface by the intravascular blood. 5. Alteration of organelies. There occured the expansion of rough endoplasmic reticulum, the degranulation and rupture of membranes, as well as the swelling and dissolution of mitochondria.