The compatibility of polystyrene (PS) and poly(methyl methacrylate) (PMMA) blends was studied over a wide range of compositions at 383, 413 and 443 K, respectively, by atomistic and mesoscopic modeling. All the calculated Flory-Huggins interaction parameters showed positive values; furthermore, they were all above the critical Flory-Huggins interaction parameter value, which means that the PS/PMMA blends were immiscible. Both the addition of a block copolymer and the introduction of a shear field influenced the phase morphologies of the blends, while the degree of influence depended on the compositions of the blends. The study of PS/PMMA blends doped with nanoparticles showed that the mesoscopic phase was influenced by not only the properties of the nanoparticles, such as their size, number and number density, but also the compositions of the blends. The effect of the surface roughness of the planes on the phase separation of the blends was also studied.