Mechanical property of four compatible blends of polystyrene / poly(phenylene oxide), polystyrene/poly(vinyl methyl ether), polystyrene/poly(α-methylstyrene). and poly (phenylene oxide / poly (a-methylstyrene) were investigated using tensile tester. Preliminary thermal and dynamic mechanical analysis showed that all four blends exhibited single compositionally dependent Tg''s. Modulus data showed synergistic effect for all blends. Break(yield) stress data exhibited linear to slight positive deviation from the rule of mixtures relationship. Percent strain at break data exhibited clear negative deviation from the rule of mixtures relationship. Percent strain at break data exhibited clear negative deviation from the rule of mixtures relationship. These mechanical property can be explained using the concept of intermolecular interaction in compatible blends. Overall, the mechanical property of four compatible blends was similar to the effect of antiplasticization.