Summary Taiwan¡¦s petroleum market has been deregulated in the wake of the passing of the Petroleum Administration Law. The market structure should have been shifted to monopolistic competition from the monopoly and the price backed to the so-called equilibrium one. Observing its historical data, we can find the effect of the price decrease is not obvious. In this article, we try to explore the reasons for that using the interest group theory in the public choice school. Every interest group demanding regulation decides how much political resource they would provide in light of their own cost benefit analysis. On the other hand, the administration department supplying regulation will be influenced by some variables such as ideology, institutional constraint, and political variance. Finally the political equilibrium price, i.e. output of regulation, will be reached through adjusting both sides each other.