In this paper, by using the linearized augmented plane wave based on the density functional theory of the first principles, the parameters of a unit cell were o ptimized for CoSi firstly. The steady state energy of a many-particle system was -134684297Ry. The corresponding volume and lattice parameter of the unit cell in the steady state were 5899360a.u.3 and 04438nm respectively. Secondly, the calculations of the electronic structures for CoSi and CoSi075A l025 were performed. At the same time, the characteristics of electronic structures of CoSi and CoSi075Al025 were analyzed. The shape of density of states of CoSi in this paper is almost the same as that of the exist ing calculations except for some differences. The Fermi level of CoSi075 Al025 shifted slightly, compared with that of CoSi. Finally, the int rinsic relations between the electronic structures of CoSi and CoSi075Al 025 and their respective thermoelectric properties were discussed in det ail. The increase of material factor B of CoSi075Al025 w as attributed to the doping with Al. Therefore, doping was a practicable means t o improve the thermoelectric property of CoSi.