The study investigates the prevalence and recidivism risk by motives of arson inmates. Survey technique is adopted. Subjects include almost all the male and female arson inmates and randomly selected non-arson inmates in Taiwan's prison, totally 630. The results show the diversity of arson motives and the characteristics of not specially distributed on some motives. Arsonists with crime concealment, sensation seeking, no special reason, hallucination responding, profit gaining, attention seeking, and vandalism motives have higher recidivism risk. The more kinds of motive the arsonists present, the higher the recidivism risk. The prevalence and recidivism risk by motives of arson inmates, structure of arson motives and types of arsonists, nature of arson behavior are discussed.