The crystal structure of Ca2Y2Si2O9 was studied by X-ray and electron diffraction. Statistical atomic site positions were determined in an orthorhombic cell, a = 10.453(3), b = 3.720(2), c = 10.290(2) Angstrom, space group Pnma. The local structure of Si2O7 silicate anions was illustrated from the refined atomic positions. The Ca2Y2Si2O9 structure could be explained by locally disordered arrangement of Y4Al2O9 monoclinic structure. The linear thermal expansion coefficient of Ca2Y2Si2O9 ceramics measured by dilatometry was 9.3 X 10(-6) degrees C-1 from room temperature to 1500 degrees C.