A new phase Na3YSi6O15 has been found in the ternary system SiO2-Na2O-Y2O3. It is Orthorhombic, space group Cmca, with cell dimensions : A = 14,744(8) angstrom; b = 17,844(8) angstrom; c = 10,610(3) angstrom; Z = 8. The structure has been refined by a least square method to a final R-value of 0,048 (Rw = 0,031) for 1536 independent reflections. The crystal structure consists of comer sharing double silicate chains with a six [SiO4]motive sequences along the c-axis.