Abstract: An additive for synthetic resins is provided which comprises particles of a petaloid hydroxyapatite represented by a chemical formula Ca10(PO4)6(OH)2, having the specified particle diameter, degree of dispersion: and specific surface area, and a synthetic resin composition blended with the additive is also provided. By the use of the additive of the present invention, a film satisfying anti-blocking property, transparency and anti-scratch property can be obtained.

Abstract: An additive for synthetic resins is provided. The additive comprises particles surface-coated with a petaloid porous hydroxyapatite having a chemical formula Ca.sub.5 (PO.sub.4).sub.3 (OH) and the petaloid porous hydroxyapatite is not less than 5% by weight of the particles. The additive can be added to any kinds of synthetic resins. If the additive is used in a polyethylene film, blocking of the film is not only prevented, but good transparency and good anti-scratch ability are imparted, and if the additive is used in a polyester, a film having good slipperiness, anti-abrasion and less coarse protrusions can be provided.