A systematic study of the structural stability,elastic properties and electronic structures for iridium silicide (IrSi) in B31 and B20-phase has been carried out with the generalized gradient approximation (GGA) and loca1 density approximation(LDA)in framework of density functional theory (DFT).The obtained formation enthalpies,energy-volume (E-V) curves and enthalpy-pressure (H-P) curves indicate that orthorhombic B31-IrSi is more stable than cubic B20-IrSi at both ambient and pressure conditions.