Pt/SiO2 composite membranes were synthesized by sol-gel process. The average particle size of the mixed sol was below 30 rim and well dispersed in the solution. TEM results show the microstructure Of Pt/SiO2 composite membranes was homogeneous. Average pore size of the Al2O3 support was about 0.125 mum and the pore size distribution was narrow. The average pore size of the Pt/SiO2 composite membranes has smaller than that of SiO2 composite membrane. It was observed that the Pt/SiO2 composite membranes showed a crack-free microstructure and narrow particle size distribution even after heat treatment up to 500 degreesC.