During 1949-1992, there were tens of thousand people in Taiwan being arrested, interrogated, and punished by the ruling authorities with the suspicion of offense of rebellion or Communist agents, resulting in white terror atmosphere's haunting Taiwan over a long period of time. On the whole, the ruling authorities' disciplinary actions for political offenders were pretty heavy; however, the illegal and inappropriate disposal was often seen, and usually caused great infringement of human rights.In the process of handling political cases, the starter of procedures of investigation was the intelligence and public security unit which usually played a crucial role. The research discussed the development and organizational chart of the intelligence and public security unit first, and then talked about intelligence agency's many illegal or inappropriate actions during the process of investigating political cases, including: illegal or inappropriate arrest, violence, threat, luring by promise of gain, deception,third degree, as well as political consideration, secret execution, arresting first and then finding the evidence, not transferring to the court within 24 hours after the arrest, improper detention, and making a feigned case, etc.