We propose an efficient bit-loading algorithm to minimize the total power to transmit a target rate of a single user in OFDM communication systems.Considering the digital modulations exploited by 802.11a,which are BPSK,QPSK,16QAM and 64QAM(constellation using Gray code and each sub-carrier carrying a maximum of 6 bits),?RT /6」subcarriers which have litter power addition are allocated with 2 bits each time.The proposed bit loading algorithm is optimal and has the same performance as Hughes-Hartogs algorithm,but greatly reduces the computational complexity.Simulation results show that the proposed bit loading algorithm has the same bit allocation results as Hughes-Hartogs algorithm,but the computational complexity is less than 50% compared with that of Hughes-Hartogs algorithm.The computational complexity will be further reduced when the transmission rate or subcarriers number increases.