In the present work, the hybridization of subnanometer-scale metal oxide cluster SiW12O404- (STA) with poly(ethylene oxide) (PEO) and PEO-block-poly(propylene oxide)-block-PEO was investigated. A new crystalline phase was formed in the hybrid nanocomposites, which is attributed to the inclusion of PEO in the lattices of STA, that is, inclusion crystallization. The crystal lattice and dimensions of STA/PEO inclusion crystals are determined to be an orthorhombic unit cell with a = 1.52 nm, b = 2.74 nm, c = 2.52 nm, and alpha = beta = gamma = 90 degrees. Remarkably, hybrid nanosheets with large size and high shape asymmetries can be formed by lamellar inclusion crystals. The dependence of proton conductivities of STA nanocomposites on crystalline structures was examined, and it was found that the formation of hybrid nanosheets could enhance proton conductivities of STA nanocomposites. This work provides new insight into the development of cluster-based nanocomposites with controlled morphology and optimized performance.