Poly(ethylene terephthalate) (PET) and polystyrene (PS) are immiscible and incompatible and have been well recognized. In this study, styrene maleic anhydride random copolymer (SMA-8 wt % MA) and tetra-glycidyl ether of diphenyl diamino methane (TGDDM) are employed as reactive dual compatibilizers in the blends of PET-PS. The epoxy functional groups of the TGDDM can react with PET terminal groups (-OH and -COOH) and anhydride groups of SMA at the interface to produce PET-co-TGDDM-co-SMA copolymers. SMA with low MA content is miscible with PS, whereas the PET segments are structurally identical with PET phase. Therefore, these in-situ-formed copolymers tend to anchor at the interface and act as effective compatibilizers of the blends. The compatibilized blends, depending on the amounts of TGDDM and SMA addition, result in smaller phase domain, higher viscosity, and improved mechanical properties. This study demonstrates that SMA and TGDDM dual compatibilizer can be utilized effectively in compatibilizing polymer blends of PET and PS.