This brief presents a system design technique for improving the energy utilization of radio frequency (RF) circuits powered by renewable energy sources. Different from conventional systems, the operation of self-powered RF circuits is largely constrained by two factors: time-varying channel conditions and nondeterministic renewable energy levels. The proposed technique dynamically adjusts the modulation scheme to deal with these two factors in a coherent manner. This is the effective way to maximize the data rate of RF circuits while maintaining the required performance under unstable energy supplies. A detailed VLSI implementation is developed with negligible energy overheads. Simulation results demonstrate that the proposed technique outperforms conventional RF circuits based on the fixed modulation scheme under variable channel and energy conditions.