Abstract: On the basis of the event-tree and probability analysis methods, a probabilistic method of nuclear terrorism risk was built, and the risk of terrorism events was analyzed. With the statistical data for and hypothetical data for relative events, the relative probabilities of the four kinds of nuclear terrorism events were obtained, as well as the relative risks of these four kinds of nuclear terrorism events were calculated by using this probabilistic method. The illustrated case show that the descending sequence of damages from the four kinds of nuclear terrorism events for single event is as following: nuclear explosive and improvised nuclear explosive, nuclear facility attacked, and “dirty bomb”. Under the hypothetical condition, the descending sequence of possibilities for the four kinds of nuclear terrorism events is as following: “dirty bomb”, nuclear facility attacked, improvised nuclear explosive and nuclear explosive, but the descending sequence of risks is as following: “dirty bomb”, improvised nuclear explosive, nuclear facility attacked, and nuclear explosive.