Core-shell structured polyacrylic (named ACR) impact modifiers consisting of a rubbery poly(n-butyl acrylate) (BA) core and a rigid poly(methyl methacrylate) shell with a size of about 310 nm were prepared by seed emulsion polymerization. The ACR modifiers with different core-shell weight ratios (85:15; 80:20; 75:25; 70:30) were used to modify the toughness of poly(butylene terephthalate) (PBT) by melt blending. It was found that the polymerization had a very high instantaneous conversion (> 90 %) and overall conversion (98 %). The ACR latexes had an obvious core-shell structure confirmed by transmission electron microscope. The mechanical properties of the PBT/ACR blends were evaluated, and scanning electron microscope (SEM) was used to observe the fractured morphology. Dynamic mechanical analysis and differential scanning calorimeter were used to study the molecular movement and crystallization behaviors of PBT/ACR blends. The results indicated that with an appropriate value of the core-shell weight ratio, poly(BA) could disperse well in the matrix and the brittle-ductile transition point could emerge. As a result, the notch impact strength of PBT/ACR blends with a core-shell weight ratio of 80:20 was 6.7 times greater than that of pure PBT, and the mechanical properties agreed well with the SEM observation.