The effects of compatibilizer on the weathering properties and hydrolysis of the poly(butylene terephthalate) (PBT)/poly(acrylonitrile‐styrene‐acrylate) (PolyASA) blend (70/30 wt %) were investigated. Poly(ethylene‐co‐glycidylmethacrylate‐g‐styrene‐co‐acrylonitrile) (EGMA‐g‐SAN) was used as a compatibilizer of the blend. When the EGMA‐g‐SAN content was 3 phr, the impact and tensile strengths of the PBT/PolyASA blend showed maximum; as the EGMA‐g‐SAN content was increased from 0 to 3 phr, the impact strength of the blend increased from 6.1 to 9.5 kJ m−2. The rheological results showed that when the EGMA‐g‐SAN was 3 phr, the storage modulus and complex viscosity of the blend also showed maximum value. The mechanical and rheological results of the blend were consistent with the morphological result that when the EGMA‐g‐SAN was 3 phr, PolyASA domain size of the blend showed minimum. The weathering results showed that when the EGMA‐g‐SAN content was 3 phr, the gloss value of weathered surface of the blend showed maximum. After hydrolysis test, when the EGMA‐g‐SAN content was 3 phr, the impact and tensile strengths of the blend showed maximum. The mechanical, rheological, and weathering results suggested that EGMA‐g‐SAN is an effective compatibilizer in the PBT/PolyASA blend, and the optimum compatibilizer content is 3 phr. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47197.