Dynamic viscoelastic properties far three kinds of miscible blends of poly(methyl methacrylate) (PMMA) and poly(styrene-co-acrylonitrile) (SAN) have been measured. It is found that the time-temperature superposition principle can be applied over the entire temperature range for the blends. The temperature dependence of the shift factors can be expressed by the WLF equation log a(T) = -8.86(T - T-s)/101.6 + T - T-s). The T-s increases linearly with an increase in the weight fraction of PMMA. The zero-shear viscosity versus blend composition curves deviate positively from linear additivity for the blends at both constant temperature and free Volume fraction. The plateau modulus, determined as the storage modulus G＇ in the plateau zone at the frequency where tan delta is at a minimum, varies linearly with blend composition for the PMMA-7 (M-w = 7.2 x 10(4))/SAN-18(M-w = 11.6 x 10(4), AN = 18.4 wt %) and PMMA-7/SAN-27 (M-w = 8.4 x 10(4), AN = 27.0 wt %) blends, but concavely with the blend composition for the PMMA-60 (M-w = 59.3 x 10(4))/SAN-18 blends. It is considered that the above method to determine the plateau modulus cannot be applicable for the PMMA-60/SAN-18 blends, because of the great difference in the concentration of entanglements. It is concluded that the relaxation behavior of the PMMA/SAN blends is similar to that of a blend consisting of homologous polymers, and the entanglement molecular weights in PMMA/SAN blends are identical with those in the component polymers.