The mechanism of biological tissue cutting was investigated using excimer laser. No or a minimal thermal effect can be observed from the post operative tissues. Many hypotheses have been published about the mechanism of tissue cutting; thermal effect, photon effect, photochemical effect, plasma effect and others. After thermographical analysis with wood and white of an egg, we have been studying this problem using rabbit's and rat's muscle and bone. Postoperative tissues were observed with light and electro microscope. Under the optimum condition, the carbonated layer couldn't be found. With light microscope and SEM, tissue fragments were ablated. With TEM, selective organelle destruction (especially mitochondoria and glycogen granule) were caused. The results show that the first step of tissue cutting mechanism using excimer laser is not due to thermal effect but due to selective organelle destruction, although the whole mechanism contains many phases.