This work is concerned with the compatibilizing efficiency of graft copolymer for the immiscible polymer blends. A series of graft copolymers of polystyrene (PS) and polyamide 6 (PA6), denoted as PS-g-PA6, with different molecular structures were used as the compatibilizers for immiscible PS/PA6 blends. The results indicated that whatever the blend composition, for PS-g-PA6 graft copolymers with the same backbone and the same number of grafts per backbone, the longer the grafts, the higher their compatibilizing efficiency; for a given backbone/graft mass ratio, the longer the grafts and concomitantly the smaller the number of grafts per backbone, the higher the compatibilizing efficiency of the graft copolymer. Moreover, a graft copolymer whose backbone and grafts match the matrix and the disperse phase polymers, respectively, has a higher compatibilizing efficiency. Molecular simulations are partly support the above results.