Disclosed is an alumina-silica resin additive consisting of amorphous alumina-silica particles in which a definite cubic or spherical shape is retained in primary particles and the particle sizes of both the primary particles and the secondary particles are controlled to very small values. This additive is excellent in dispersibility in a resin, non-blowing property, slip characteristic and anti-blocking property. A product obtained by surface-treating the amorphous alumina-silica particles with a certain amount of an organic lubricant is very valuable as a filler.