Projective synchronization phenomenon in unidirectionally coupled unified systems is studied- A scheme is proposed to control the scaling factor formed in the projective synchronization- Under the control strategy, one can arbitrarily mani pulate the scaling factor to a desired value- This special phenomenon and the co ntrol scheme can be used in some secure communication schemes- Illustrations ar e also given to show the rightness of the theoretical analysis and the effective ness of the method proposed-