Magnesium Silicide is one of the interesting thermoelectric materials known for relative abundance of its constituents, thermal stability, non-toxicity and environmental friendly nature. In this paper we have theoretically studied Bi doped Mg_2Si. The electronic structure calculations predict non-existence of the energy gap for Mg_2Si_(1-x)Bi_x with(0.125≤ x≤0.5). It has been found that the system with x=0.125 exhibits highest Seebeck coefficient and electrical conductivity. Due to low thermal conductivity at x=0.125, Mg_2Si_(0.875)Bi_(0.125) attained maximum value of dimensionless figure of merit 0.67 at 1200 K. With increase in concentration of Bi,the value of figure of merit decreases. Magnesium Silicide is one of the interesting thermoelectric materials known for relative abundance of its constituents, thermal stability, non-toxicity and environmental friendly nature. In this paper we have theoretically studied Bi doped Mg_2Si. The electronic structure calculations predict non-existence of the energy Due to low thermal conductivity at x = 0.125, Mg 2 Si 0.875 (0.875), the lattice constant of Mg 2 Si_ (1-x) Bi_x with (0.125≤x≤0.5) ) Bi_ (0.125) attained maximum value of dimensionless figure of merit 0.67 at 1200 K. With increase in concentration of Bi, the value of figure of merit decreases.