Poly(vinyl pyrrolidone-co-vinyl acetate)-graft-poly (epsilon-caprolactone) (PVPVAc-g-PCL) was synthesized by radical copolymerization of N-vinyl-2-pyrrolidone (VP)/vinyl acetate (VAc) comonomer and PCL macromonomer containing a reactive 2-hydroxyethyl methacrylate terminal. The graft copolymer was designed in order to improve the interfacial adhesiveness of an immiscible blend system composed of cellulose acetate/poly(epsilon-caprolactone) (CA/PCL). Adequate selections of preparation conditions led to successful acquisition of a series of graft copolymer samples with different values of molecular weight (M-n(PVPVAc-g-PCL)), number of grafts (n), and segmental molecular weight of PVPVAc between adjacent grafts (M-n (between grafts)). Differential scanning calorimetry measurements gave a still immiscible indication for all of the ternary blends of CA/PCL/PVPVAc-g-PCL (72 : 18 : 10 in weight) that were prepared by using any of the copolymer samples as a compatibilizer. However, the incorporation enabled the CA/PCL (4 : 1) blend to be easily melt-molded to give a visually homogeneous film sheet. This compatibilizing effect was found to be drastically enhanced when PVPVAc-g-PCLs of higher M-n(PVPVPAC-g-PCL) and M-n (between grafts) and lower n were employed. Scanning electron microscopy revealed that a uniform dispersion of the respective ingredients in the ternary blends was attainable with an assurance of the mixing scale of several hundreds of nanometers. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 2945-2954, 2009