A generalized projective synchronization method is proposed based on unidirectional coupled synchronization by only transmitting a single driving variable. The proposed method allows us to arbitrarily direct the scaling factor to a desired value. Since chaos synchronization can be achieved by transmitting a single variable from driving system to response system, this method is more practical. Theoretical deduction and computer simulation show the effectiveness of the proposed method. A scheme for secure communication, which can increase the security of a communication system, is presented based on generalized projective synchronization of unified chaotic system.