Abstract: A new method, first order derivative spectrophotometry, for determining the encapsulation ratio of camptothecin in the polyphase liposome is introduced in this article. Compared with others, this method has many advantages such as simple, correct, fast, etc. The size and shape do not interfere with the accuracy of the result. The encapsulation ratio determined with this method is 73.0±3.4% (n=5). Using this method, we have studied the leaking property of camptothecin in the polyphase liposome. The results indicate that the leakage speed of camptotthecin from the polyphase liposome is easily influenced by temperature, pH, medium, and by the extent of oxidation of the membrane components.