The in situ polymerization of methyl methacrylate (MIMA) with poly(styrene-co-acrylonitrile) (SAN) was studied. The PMMA/SAN in situ polymerization blends obtained were examined by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), tensile tests and scanning electron microscopy (SEM). The blends with compositions of 95/5, 80/20, 70/30, and 60/40 in weight ratios were miscible and had a single phase structure. However, the 90/10 PMMA/SAN in situ polymerization blend obtained was inhomogeneous and had a two-phase structure; polymerization-induced phase separation occurred during the preparation process of the blend. Both tensile strength and elongation at break increase with increasing SAN content up to 30 wt %. The degradation temperature and thermal stability of PMMA increased remarkably on incorporation of SAN up to 30 wt %.