The miscibility of binary polymer blends of polystyrene/poly(styrene-co-acrylonitrile) (PS/SAN) was predicted based on both Flory's equation of state theory (EOS) and Wolf's theory on binary polymer blends of the A/A-B type. The calculating results suggested that PS/SAN blend exhibits the upper critical solution temperature (UCST) behavior within acrylonitrile's volume fraction range of 4-20%. PS/SAN blends were studied with the Small Angle Light Scattering (SALS) method to test the theoretical prediction. It showed that PS/SAN (AN:20 vol.%) blend (49/51 wt.%) displays UCST behavior and the UCST temperature was about 171 8C. The calculating and the experimental results were in accordance with each other relatively well.