In this work, the polystyrene/poly(methyl methacrylate) (PS/PMMA) blends and their composites with different PS/PMMA ratios and different carbon black (CB) contents were prepared by melt mixing. The rheological properties of immiscible PS/PMMA blends and PS/PMMA/CB composites are deeply discussed to understand the relationship between the rheological properties and the CB particle network. For the blends with a sea-island morphology, a shoulder is observed at low-frequency region due to the shape relaxation of dispersed droplets. However, for the blends with a co-continuous morphology, the frequency sweep exhibits power-law dependences with a small terminal slope. As the CB concentration increases, the slope of storage moduli versus angular frequency plots in the low angular frequencies becomes much lower than 1. This suggests that the composite exhibits a transformation from liquid-like to solid-like states. Moreover, a linear relationship between the storage moduli at zero stress and the CB concentration in the PS phase (real CB concentration) is proposed regardless of the PS/PMMA ratio.