In recent years, energy-efficiency has become one of the most critical issues in wireless communications. Although it is important and practical to design an energy-efficient power-loading algorithms for uniform-rate orthogonal frequency division multiplexing (OFDM) systems, the problem has not been explored in literature. The terminology uniform-rate in this paper means that each subcarrier adopts the same modulation scheme. To this end, we propose an adaptive power-loading algorithm to maximize the energy-efficiency for a uniform-rate OFDM system in this paper. To reduce the complexity, a sub-optimum solution is proposed. The numerical results show that the sub-optimum solution is close to the optimum solution. We also show that by carefully determining the modulation scheme, energy efficiency can be extensively improved.