The phase behavior of polystyrene and poly(n-pentyl methacrylate) (PS/PnPMA) blends with small amounts of symmetric PS-block-PnPMA copolymer (PS-b-PnPMA) was studied by turbidity, light scattering, and small-angle neutron scattering (SANS). At ambient pressure, PS/PnPMA with lower molecular weights exhibited both the upper critical solution transition (UCST) and the lower critical solution transition (LCST), and with increasing molecular weights of PS, an hourglass type of phase behavior was observed. When a very small amount (similar to0.5 wt %) of symmetric PS-b-PnPMA is added to the blend, LCST increases but UCST decreases. This indicates that the miscibility window is expanded, and the PS-b-PnPMA acts as an effective compatibilizer in the PS/PnPMA blend. However, when the amount of symmetric PS-b-PnPMA is larger than a critical amount which depends on the molecular weight of block copolymers, the turbidity temperatures (T-b) for the LCST are essentially the same regardless of blend compositions. Thus, the turbidity temperature for the LCST of an asymmetric blend composition (for instance, 20/80 (w/w) or 80/20 (w/w) PS/PnPMA blend) with a symmetric PS-b-PnPMA was lower than that for another blend without the block copolymer, suggesting that the block copolymer does not act as a compatibilizer for asymmetric blend compositions. This interesting phase behavior was discussed in terms of the segmental interaction parameter (chi) measured by SANS and compared with predictions by the incompressible mean-field theory and the lattice cluster theory.