Novel organic-inorganic hybrid materials were prepared by in situ polymerization of silicon alkoxide in a phenolic resin matrix. Very uniform hybrid materials composed of a phenolic resin and silica were obtained using varying amounts of silica. The transparency of the hybrid materials could be varied depending on the size of the resulting silica particles. SEM observations revealed that the hybrid materials consist of fine silica particles embedded in a phenolic resin matrix with good interaction at the interface. Density and Si-29-NMR measurements indicate that the silica incorporated in the hybrid material has a high density and mainly Q4 chemical bonding environments. The hybrid material exhibits excellent mechanical improvements in modulus, strength, strain at break and impact strength.