The preparation of poly(n-butyl acrylate)/poly(acrylonitrile-co-styrene), i.e., poly(BA)/poly(AN-co-St) (PBAS) core-shell structured modifier with controlled particle size was reported, and the mechanical properties of AS/PBAS blends were investigated. The modifier was prepared at a solid content of 50 wt % by a two-stage sequential emulsion polymerization. Dynamic light scattering (DLS) was used to monitor the particle diameters and showed that the particles grew without significant secondary nucleation occurring. The morphology was confirmed by means of transmission electron microscopy (TEM). According to the research on mechanical properties of the AS/PBAS blends, a remarkable toughening effect of PBAS on AS resin was found. By means of scanning electron microscopy (SEM) observation, the toughening mechanism was proposed to be crazing caused by rubber particles and shear yielding of AS matrix. Uniform dispersion of rubber particles in AS matrix was attributed to the good compatibility between AS and PBAS modifier. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012