The influence of adding nanoparticles on the ascast morphology of spin coated immiscible polystyrene/poly(methyl methacrylate) (PS/PMMA) thin films of different thickness (h(E)) and composition (R-B, volume ratio of PS to PMMA) has been explored in this article. To understand the precise effect of nanoparticle addition, the morphology of PS/PMMA thin blend films spin cast from toluene on a native oxide covered silicon wafer substrate was first investigated. It is seen that in particle free films, the generic morphology of the films remains nearly unaltered with increase in h(E), for R-B = 3:1 and 1:3. In contrast, strong h(E) dependent morphology transformation is observed in films with R-B = 1:1. Subsequently, thiol-capped gold nanoparticles (AuNP) containing films with different particle concentrations (C-NP) were cast from the same solvent along with the polymer mixture. We observe that addition of AuNPs barely alters the generic morphology of the films with R-B = 3:1. In contrast, the presence of the particles significantly influences the morphology of the films with R-B = 1:1 and 1:3, particularly at higher C-NP (approximate to 10.0%). X-ray photoelectron spectroscopy and X-ray reflectivity of some samples reveal that the AuNPs tend to migrate to the free surface through the PS phase, thereby stabilizing this layer partially or fully (depending on C-NP) against dewetting over a surface of adsorbed PMMA layer and influencing the ascast morphology as a function of C-NP. The work is fundamentally important in understanding largely overlooked implications of nanoparticle addition on the morphology of PS/PMMA blend thin films which forms the fundamental basis for future interesting studies involving dynamics of nanoparticles within the blend thin films.