Suppose a case like this:once a patient get a serious disease,which is difficult to be cured or even can not be cured,then he or she go to a hospital to seek help,but suffered with the omission of the doctor. Most patients can not get compensation, for the reason that the patient can not prove the causation between the omission and the damage. Lots of researchers know that is unreasonable, then loss of chance doctrine is developed. Under this doctrine, one theory propose to change the traditional “damage”, and recognizes the lost chance of patients as the damage in the tort law, besides, it complies with the “but for” approach and the balance of probability. In the judicial practice, the doctrine was modified. There are two theories, one is substantial probability that lower the standard of the balance of probability, the other is proportional approach. This article analyzed loss of chance doctrine. It should provide the model and the reference when our country judicial processed similar medical service disputes.