A new modulation scheme called dual duration pulse position modulation (DD-PPM) is proposed for optical wireless communication system to overcome the short comings of pulse position modulation (PPM) and digital pulse interval modulation (DPIM). After introducing symbol structure, we study the bandwidth requirement, average transmission power and capacity. Meanwhile, the packet error rate in weak turbulence channel model is derived, and compared with on-off keying (OOK), PPM and DPIM. Theoretical and simulation results show that DD-PPM requires less average optical power and gives better bit error performance compared with OOK, and offers high bandwidth efficiency and transmission capacity compared with PPM for the given parameters. Because the symbol length is fixed in DD-PPM, there is no problems such as buffer overflow or waiting in modem, and it is more realizable than DPIM. Hence, DD-PPM is superior in optical wireless communication system.