We investigate self-assembly of nanocomposite films composed of lameller-forming poly(styrene-b-methyl methacrylate) (PS-b-PMMA) and PMMA-grafted magnetite (Fe3O4) nanoparticles (NPs). The Fe3O4 NPs are grafted with PMMA brushes with molecular weights ranging from 2700 to 35 700 g/mol. For the NP with the lowest molecular weight rush, the morphology of the nanocomposite film depends oil the NP concentration (phi(NP)). At low phi(NP), the block copolymer self-assembles into mixed morphology of perpendicular lamellae (Lam) and parallel lamelllae (IILam), and the perpendicular to Lam are stabilized by individual NPs or small NP aggregates. The NPs Call also retard the dynamics of self-assembly of block copolymer films. At high phi(NP), NPs form small aggregates which inhibit the formation of a lamellar structure. As the molecular weight of the PMMA brush increases to 13 300 or 35 700 g/mol. the Fe3O4 NPs form aggregates in the as-cast nanocomposite films, and this behavior is attributed to aggregation of NPs in the solution state. Since the size of NP aggregates is larger than the copolymer domain size, the block copolymer his to self-assemble around these aggregates. The magnetic properties of these nanocomposite films are characterized, and typical superparamagnetic behavior is observed.