Well-defined poly(styrene-g-caprotactone) copolymers (PS-g-PCL) were synthesized and they were used as the compatibilizers for polycarbonate (PC) and polystyrene (PS) blends. For the PS-g-PCL graft copolymers, polycaprolactone macromer having M,, of 8,700 g/mole and polydispersity of 1.07 was copolymerized with styrene via radical polymerization. The PS/PCL weight ratio of the three obtained graft copolymers was 50/50, and the number of PCL branches per polymer chain were 3, 6, and 8. Thermal analysis showed that the three graft copolymers contain crystalline PCL phase. Scanning electron microscopy (SEM) pictures showed that the domain size of PS decreased when the contents of the graft copolymer containing 3 branches increased from 1 to 5 wt% in PC/PS ,)0170 wtfc blends. A weak melting endotherm on the differential scanning calorimetric analysis indicates that not all PCL branches are situated on the PC/PS interface when the content of the graft copolymer is more than 3 wt%. When 5 wt% of each graft copolymer wa.1 blended, the sizes of PS domain of the three PC/PS 30/70 wt% blends were similar but slightly larger when the number of branch of graft copolymer was 8.