Abstract: The electronic, mechanical and optical properties of  Si_8-xGe_x alloys were investigated by using firstprinciples calculation method based on density functional theory. The calculation results show that the structures of energy bands of  Si_8-xGe_x alloy are similar, the elastic coefficient, bulk modulus and shear modulus decrease with the increase of x, the 
static dielectric constants, refractive index and reflection coefficient increase quasilinearly with the increase of x, 
and the plasma oscillation frequency decreases with the increase of x.

Key words: Si_8-xGe_x alloy, electronic property, optical property, firstprinciples calculation