We investigated the influence of the chain asymmetry of the styrene (S)-methyl methacrylate (MMA) copolymer (surfactant) on the morphology of the polystyrene (PS)/poly(methyl methacrylate) (PMMA) blends. It was, for the first time, demonstrated that the chain asymmetry of the block copolymer surfactant would have significant influence on the structure of the polymer blend in the melt mixing process. In the PS/PMMA = 30/70 blends, the addition of asymmetric AB diblock copolymers tuned the morphology from the droplet-in-matrix structure to the cocontinuous structure. At 20 wt % asymmetric AB block copolymer (PMMA(110)-block-PS430) dosage, the onset of the cocontinuous interval of the PS/PMMA blends was lowered to PS/PMMA = 20/80 and the cocontinuous interval was broadened to PS/PMMA = 20/80-30/70, compared with the onset at PS/PMMA = 45/55 and the interval at PS/PMMA = 45/55-50/50 of the PS/PMMA blends without the copolymers. The reason was that the packing of the asymmetric block copolymer on the interface provided the bending energy being against the interfacial energy when the longer block of the copolymer was in the minor phase side, suppressing the breakup of the minor phase fibers during melt mixing. With the addition of 20 wt % AC block copolymer (P(S-ran-MMA)(220)-block-PS320), which was more asymmetric than the AB block copolymer (PMMA(110)-block-PS430) of the same molecular weight and monomer composition, the PS/PMMA blends showed an even wider cocontinuous interval at PS/PMMA = 10/90-30/70.