The pH value and viscosity of Y_2O_3–SiO_2 (Y–Si) slurry made by Y_2O_3 powders and silica sol for the face coat of Ti–6Al–4V investment casting were measured. The thermal behavior of the shell made by the Y–Si face coat system was investigated by differential scanning calorimeter(DSC), thermal gravimetric(TG) analysis combined with mass spectrometry(MS), and the phase transformations were determined by X-ray diffraction(XRD). Hot strength, residual strength, linear expansion coefficient, and wearing resistance performance of the shell were also tested. The microstructure and elements distribution of the interaction layer were studied by scanning electron microscope(SEM) and energy-dispersive spectrometer(EDS), respectively. The microhardness tester was applied for the microhardness. The results showed that the slurry was stable for at least 60 h. A very small amount of YZrO_3 was formed below 1050 °C and Y_2 SiO_5was formed around 1450 °C. The shell made by Y–Si system had good mechanical property which could reduce cracks during the procedure of dewaxing and inclusions during pouring.Some Al volatilized from the melt, permeated the surface of the face coat shell, and formed the black reaction layer,which blocked the permeation of O so that O penetration was limited to 5 lm. The depth of Si penetration was about 60 lm. The hard layer was also around 60 lm. The thermal behavior of the shell made by the Y-Si face (Y-Si) slurry made by Y_2O_3 powders and silica sol for the face coat of Ti-6Al-4V investment casting were. coat system was investigated by differential scanning calorimeter (DSC), thermal gravimetric (TG) analysis combined with mass spectrometry (MS), and the phase transformations were determined by X-ray diffraction The microstructure and elements distribution of the interaction layer were studied by scanning electron microscope (SEM) and energy-dispersive spectrometer (EDS), respectively. The microhardness tester was applied for the microhardness. The results showed that the slurry was stable for at least 60 h. A very small amount of YZrO_3 was formed below 1050 ° C and Y_2 SiO_5 was formed around 1450 ° C. The shell made by Y-Si system h ad good mechanical property which could reduce cracks during the procedure of dewaxing and inclusions during pouring.Some Al volatilized from the melt, permeated the surface of the face coat shell, and formed the black reaction layer, which blocked the permeation of O so that O penetration was limited to 5 lm. The depth of Si penetration was about 60 lm. The hard layer was also around 60 lm.