Thermoelectric transport properties of Pb doped SnSe alloys, PbxSn1-xSe, were systematically investigated by DFT-BTE simulations. The figure of merit is mainly enhanced by reducing lattice thermal conductivity which is estimated about 43% decrease with Pb addition. Electrical conductivity and Seebeck coefficient evaluated by using the Boltzmann transport equation with a constant relaxation time approximation. These electronic transport properties hardly changed with Pb addition, and did not make any significant change in electron transport. However, the considerable change in phonon transport and lattice thermal conductivity are shown. This investigation elucidates the electrons and phonons transport phenomena in alloys, leading the design of new high-performance thermoelectric materials.