Abstract: A method for preparing a stable silica sol having an SiO.sub.2 concentration of 40% or less, the colloidal particles of said silica having a particle size of 40 to 500 nm, as measured by dynamic light-scattering and having an elongated shape having a thickness of 5 to 40 nm and an elongation of 5 to 30 times the thickness in only one plane, the method comprising the steps (a), (b) and (c): (a) mixing an aqueous solution of Ca or Mg salt with an aqueous solution of active silicic acid having an SiO.sub.2 concentration of 1 to 6% and a pH value of 2 to 5, or with an aqueous acidic silica sol having a particle size of 3 to 30 nm, an SiO.sub.2 concentration of 0.5 to 25% and a pH value of 1 to 5 in a weight ratio of 1500 to 10000 ppm of (CaO or MgO)/SiO.sub.2 ; (b) mixing an alkali metal hydroxide with the solution obtained by step (a) in a molar ratio of 20 to 300 as SiO.sub.2 /M.sub.2 O, wherein M represents an alkali metal atom; and (c) heating the mixture obtained by step (b) at 60.degree. to 300.degree. C.