We report a greatly enhanced thermoelectric performance in a BiCuSeO system, realized by improving carrier mobility through modulation doping. The hetero-structures of the modulation doped sample make charge carriers transport preferentially in the low carrier concentration area, which increases carrier mobility by a factor of 2 while maintaining the carrier concentration similar to that in the uniformly doped sample. The improved electrical conductivity and retained Seebeck coefficient synergistically lead to a broad, high power factor ranging from 5 to 10 mu W cm(-1) K-2. Coupling the extraordinarily high power factor with the extremely low thermal conductivity of similar to 0.25 W m(-1) K-1 at 923 K, a high ZT approximate to 1.4 is achieved in a BiCuSeO system.