The mechanical and heat-resistant properties of acrylonitrile-butadiene styrene (ABS) binary and ternary blends were investigated. The relationship of compatibility and properties was discussed. The results show that poly(methyl methacrylate) (PMMA) and styrene-maleic anhydride (SMA) can improve the thermal properties of conventional ABS. The Izod impact property of ABS/PMMA blends increases significantly with the addition of PMMA, whereas that of ABS/SMA blends decreases significantly with the addition of SMA. Blends mixed with high-viscosity PMMA are characterized by higher heat-distortion temperature (HDT), and their heat resistance is similar to that of blends mixed with SMA. For high-viscosity PMMA, from 10 to 20%, it is clear that blends appear at the brittle-ductile transition, which is related to the compatibility of the two phases. TEM micrographs show low-content and high-viscosity PMMA in large, abnormally shaped forms in the matrix. Compatibility between PMMA and ABS is dependent on both the amount and the viscosity of PMMA, When the amount of high-viscosity PMMA varied from 10 to 20 wt %, the morphology of the ABS binary blends varied from poor to satisfactory compatibility. As the viscosity of PMMA decreases, the critical amount of PMMA needed for the compatibility of the two phases also decreases. SMA as a compatibilizer, improved the interfacial adhesiveness of ABS and PMMA, which results in PMMA having good dispersion in the matrix.