The study on synthesis process and structure of silica-gel particles modified Al / Mg has been achieved. The modified silica gel has been expected for functional materials and catalysts. An atomizer is used for particle production of silica gel. But, many hollow particles which were easily destroyed by handling were produced in the atomizer process. The number of those weak particles decreased with increasing the viscosity of silica - sol which could be controlled by treated in relatively unstable state such as pH = 2 and 40°C , 50h. Small silica particles in silica sol was thought to bind each other during the treatment and form apparently larger molecular weight silica - sol chain. In subsequently drying process, Al and Mg nitrates which had been mixed with silica sol changed partly to the oxides such as MgAl2O 4 and MgO and partly to the elements which constructed the modified silica - gel. The chemical and physical structure of the calcinated modified silica gel were stable in the region from 450 °C to 700 °C. Half of Al and quarter of Mg were in modified silica gel structure, where (Si -O)n - Al·Mg2+, MgAl2O 4, and MgO were observed by thermal analyses (TG, DTA) and NMR. The chemical structure as well as physical structure was unstable at more than 700 °C. Pore volume rapidly decreased and the coordination number of Al fluctuated between four to six from 700 °C to 1,000 °C. The synthesis process and the structure of Al / Mg modified silica gel were partly clarified.