A new approach to loading for orthogonal frequency division multiplexing (OFDM) system is proposed, this bit-loading algorithm assigns bits to different subchannels in order to minimize the transmit energy. In the algorithm,first most bit are allocated to each subchannels according to channel condition, Shannon formula and QoS require of the user, then the residual bit are allocated to the subchannels bit by bit. In this way the algorithm is efficient while calculation is less complex. This is the first time to load bits with the scale following Shannon formula and the algorithm is of O (4N) complexity.