Based on state-observer,projective synchronization in a general class of chaotic system is studied,where the corresponding variables between the drive system and the response system are proportional. From the stability of the system,the controller is derived through the pole placement technique by designing the state-observer of the drive system. The projective synchronization could be realized in a general class of discrete systems. It allows us to arbitrarily amplify or reduce the scale via a feedback control on the slave system in controlling chaos. It is shown that the phase space of response system can be expanded or compressed flexibly as demanded by adjusting the scaling factor arbitrarily. This property makes it potentially useful in secure communication. In addition,the Henon map and the hyperchaotic generalized Henon map are used to show the rightness of the theoretical analysis.