The effect of adding 0.10-0.30 wt%Si on microstructure of the yttrium modified alloy IC6 was examined by scanning electron microscope (SEM), energy dispersive spectrum(EDS) technique of electron probe micro-analyzer (EPMA) and transmission electron microscope (TEM). The results show that two bulk shape phases, Mo1.24Ni0.76 and Mo-6(Ni0.75Si0.25)(7) are formed in the interdendritic area in the alloy with addition of 0.10-0.20 Siwt% and 0.12 wt%Y, and that a needle like phase named Y-NiMo precipitates in the interdendritic area in the alloy with addition of 0.30 wt% silicon and 0.12 wt% yttrium besides the formation of the bulk shape phases mentioned above. The stress rupture properties under 1100 degreesC/80 MPa were improved by adding 0.12 wt%Y but decreased by adding 0.10-1.30 wt%Si and 0.12 wt%Y.