In communication for the feedback control of a noiseless dynamical system, adaptive modulation is studied to adapt the modulation scheme to the physical system state or parameters in networked control systems. The impact of delay and packet drop rate incurred by modulation is studied. Then, the adaptive modulation is formulated as optimization problems for the scenarios in which the modulation is adaptive to either the system parameters or system state. Numerical simulations in the context of smart grids demonstrate the validity of the proposed adaptive modulation scheme.