Abstract: In free space optical communications, in order to get the self-symbol synchronization and fixed symbol length, a novel flag dual amplitude pulse position modulation(FDAPPM) was proposed. The symbol structure, transmission rate and error probability of the FDAPPM system over atmospheric turbulence-induced fading channel were presented. By using the maximum-likelihood sequence detection, the high SNRs asymptotic on the error probability of the FDAPPM was analyzed and the diversity order and SNR modulation gains were calculated. The performance of the FDAPPM was compared with the conventional OOK, PPM, FDPIM, FDAPIM and FDAPPM. Numerical results indicate that under the same turbulence condition, the FDAPPM possesses the advantages over the FDPIM and FDAPIM in terms of transmission rate and error performance. It also provides high transmission rate than the PPM and simplifies the receiver design owing to the built-in symbol synchronization. Moreover, it does not have insertion/deletion error in modem due to the fixed symbol length.