We have demonstrated the effects of different chain lengths on the phase behavior in films of binary blends of four different weight fractions in a weak segregation limit. The binary blends were composed of nearly symmetric polystyrene-block-poly(methyl methacrylate), PS-b-PMMA, and homo-polystyrenes (PS) of three different molecular weights (hereafter hPS2.8, hPS6, and hPS17). Films of two initial thicknesses were prepared by spin coating of the blends. After thermal annealing at 270 °C (1 h), a series of nanodomains, including perforated layers, double gyroid, cylinders, and spheres, were obtained in sequence as the volume fractions of PS increased in blend films. Perforated layers and double gyroid only exist in PS-b-PMMA-rich blend films, with hPS2.8/PS-b-PMMA blends overwhelmingly favoring perforated layers, regardless of film thickness. However, with a small amount of hPS6chains, PS-b-PMMA-rich blends preferentially form hexagonally perforated layers in thin films but double gyroid in thick films; at the same composition, PS-b-PMMA-rich blends with hPS17chains only form hexagonally perforated layers. Cylinders only formed in blend films of equal weight fractions of PS-b-PMMA and PS. The cylinders with hPS2.8and hPS6chains favor parallel orientation, but cylinders with hPS17PS chains favor a mixed orientation. Spheres predominantly formed in PS-rich blend films, where four types of domain ordering (hexagonal packing, body-centered cubic (BCC), face-centered cubic (FCC), and short-range order) were obtained. For blend films prepared in the PS-rich regions, the four types of ordering depend on chain length and film thickness. The PS-b-PMMA blend films with hPS2.8chains predominantly favor hexagonal packing of spheres for thin films but BCC packing of spheres for thick films. Under the same PS-rich region, the PS-b-PMMA blend films with hPS6chains predominantly favor hexagonal arrays of spheres in thin films but FCC-packed arrays of spheres in thick films. The PS-b-PMMA blend films with hPS17chains predominantly favor short-range-order spheres or a mixture of spheres and cylinders with no lattice packing. All of the above findings are explained by the correlation between the variations in chain length and alleviating packing frustration.