High performance n-type bulk BiAgSeS is successfully synthesized to construct heterogeneous composites which consist of mesoscale grains of both pristine BiAgSeS and doped BiAgSeS1-xClx (x = 0.03 or 0.05). Without perceptibly deteriorating the Seebeck coefficient, a significant enhancement on electrical conductivity is obtained due to an anomalous increase of both carrier mobility and concentration; the enhanced carrier mobility is proven to be a direct result of modulation doping which relates to the band alignments, while the increased carrier concentration is attributed to the possible charge transfer from Cl rich nanoscale precipitates at the heterogeneous BiAgSeS/BiAgSeS1-xClx grain boundaries. Eventually, an enhanced figure of merit ZT approximate to 1.23 at 773 K in the composite (BiAgSeS)(0.5)(BiAgSeS0.97Cl0.03)(0.5) is achieved, indicating that heterogeneous composites ultilizing the mechanism of modulation doping shall be a promising means of boosting the performance of thermoelectric materials. This strategy should be very likely applicable to other thermoelectrics.