Disclosed is a composition for the inhibition of transplant rejection and the treatment of skin diseases, comprising a cordyceps mycellia extract as an active ingredient. The cordyceps mycellia extract significantly suppresses the production of antibodies to transplants without side effects, such as weight change. Based on natural material, the composition is non-toxic and harmless to the human body and thus can be used as an immunosuppressant for organ transplantation. Also, it stops oozing from sores and is useful in the treatment of skin diseases, including atopy, allergic reactions, decubitus ulcers, pemphigus and smallpox.