ABSTRACT Herein, poly(styrene‐ co ‐acrylonitrile)/poly(methyl methacrylate) (SAN/PMMA, 70/30 wt%) blends with the PMMA matrix of various molecular weights were prepared, and their miscibility was investigated at 180°C and 210°C. The rheological analysis of Cole–Cole and van Gurp‐Palmen plots confirmed that in the case of the blends obtained with smaller molecular weights of PMMA (10 kDa, 50 kDa), phase separation occurred only at 210°C. The small difference between the viscosity of PMMA with a higher molecular weight (130 kDa) and SAN was responsible for the stable viscoelastic behavior of that blend at both temperatures. To overcome the observed immiscibility, hybrid particles of graphene oxide (GO) with well‐defined bimodal PMMA chains tethered on their surface, GO‐ g ‐PMMA(10 kDa/190 kDa), were prepared using surface‐initiated atom transfer radical polymerization (SI‐ATRP) and tested as a compatibilization agent. Addition of GO‐ g ‐PMMA (1 wt%) to the polymer matrix resulted in a better dispersion of particles and chain entanglements between the chains tethered on the particles and the matrix. It was determined that the best compatibilizing effect, leading to the miscible SAN/PMMA blends at both 180°C and 210°C, was observed for blends with the smallest molecular weight of the PMMA matrix, SAN/PMMA(10 kDa) containing bimodal hybrid particles.