Polymer nanocomposites (PNCs) that employ diblock copolymers (BCPs) to organize and align anisotropic nanoparticles (NPs) have the potential to facilitate self-assembling hierarchical structures. However, limited studies have been completed to understand the parameters that guide the assembly of nonspherical NPs in BCPs. In this work, we establish a well-defined nanoplate system to investigate the alignment of two-dimensional materials in a lamellar-forming poly(styrene-b-methyl methacrylate) (PS-b-PMMA) BCP with domains oriented parallel to the substrate. Monodisperse gadolinium trifluoride rhombic nanoplates doped with ytterbium and erbium [GdF3:Yb/Er (20/2 mol %)] are synthesized and grafted with phosphoric acid functionalized polyethylene glycol (PEG-PO3H2). Designed with chemical specificity to one block, the nanoplates align in the PMMA domain at low volume fractions (ϕ = 0.0083 and ϕ = 0.017). At these low NP loadings, the BCP lamellae are ordered and induce preferential alignment of the GdF3:Yb/Er nanoplates. However, at high volume fractions (ϕ = 0.050 and ϕ = 0.064), the BCP lamellae are disordered with isotropically dispersed nanoplates. The transition from an ordered BCP system with aligned nanoplates to a disordered BCP with unaligned nanoplates coincides with the calculated overlap volume fraction, ϕ* = 0.051, where the pervaded space of the NPs begins to overlap. Two phenomena are observed in the results: the effect of lamellar formation on nanoplate orientation and the overall phase behavior of the PNCs. The presented research not only expands our knowledge of PNC phase behavior but also introduces a framework to further study the parameters that affect nanoplate alignment in BCP nanocomposites. Our ability to control anisotropic NP orientation in PNCs through self-assembling techniques lends itself to creating multifunctional materials with unique properties for various applications such as photovoltaic cells and barrier coatings.