This paper presents a new approach to reduce finite state machines with respect to a CTL formula to alleviate state explosion problem. Reduction is achieved by removing parts useless to the formula of original machines. The main contribution of this paper is to exploit relations among subformulas of the CTL formula so as to gain more reduction, as well as to extend traditional pruning method, which handles only existential formulas, to handle universal formulas. Experimental results show the effectiveness of the approach.