The purpose of this paper is to deal with the syntactic properties of PRO on the basis of Control Theory. Chapter 2 is to distinguish obligatory control from optional(arbitrary) control by analyzing the syntactic relation between the controller of PRO in the matrix sentence and the function of PRO in the infinitive clause. And PRO must be c-commanded by the obligatory control but can't necessarily be c-commanded by the optional control. But c-command can't be an absolute condition in discriminating the control types of PRO.