Styrene-acrylonitrile copolymer(SAN) is a useful product with broad applications, but it has a critical drawback, brittleness. The laborious studies have been carried out to improve the impact strength of SAN, resulting in development of rubber toughened plastics which can be prepared by inclusion of rubber particles into SAM matrix. Acrylonitrile-butadiene- styrene terpolymers (ABS) are typical rubber toughened plastics but have low resistance to W light due to the double bond in butadiene rubber. In this study the butadiene rubber was replaced by acrylate rubber to prepare acrylonitrile-styrene- acrylate terpolymer (ASA). Butyl acrylate was used as a monomer and ethyl acrylate or 2-ethyl hexyl acrylate was incorporated as a comonomer to obtain the base rubbers, which would be grafted with SAW. The gel content of the base rubber was the highest when the crosslinking agent concentration was 5wt% to the monomer. The effects of the crosslinking agent concentration, the chain transfer agent concentration, and the monomer to polymer ratio on the graft ratio of ASA and the viscosity of free SAU were investigated in detail. The impact test showed that ASA had the highest toughness when the crosslinking agent concentration was 3wt% and the monomer to polymer ratio was 6/4. The impact strength of various resins which had different base rubbers was measured in the range of -40~$25^{circ}C$.